feat: let a module route stream an image-sized upload, explicitly and bounded

ADR 0017 capped a module route at 1 MiB and said large media belonged to a
future storage design. That design arrived — ADR 0030 gave modules a bucket,
ADR 0031 took Ekklesia out of the read path — and it did not solve the write
path: signed write URLs were deliberately declined, so the only way bytes
reach a bucket is a module's own route, and a photograph is not 1 MiB.

Extend the route contract rather than adding a second HTTP system. A route
declares `bodyMode`: `buffered` (the default, and what every existing route
already meant) keeps the 1 MiB ceiling and materializes the body before the
handler runs; `streamed` raises the ceiling to 32 MiB and counts bytes as the
handler consumes them, holding nothing. The mode is explicit rather than
inferred from the size, because inferring it would mean an author who raised a
limit silently changed how their own handler receives its body and when a 413
can reach their own code.

32 MiB holds one 25 MiB image plus bounded overhead and is also the platform's
absolute ceiling: no route in any mode may declare more, and no unbounded
option exists in the contract. The unit is one file per request — twenty
images are twenty individually bounded requests, and nothing here can express
a 500 MiB aggregate whose failure would waste everything already sent.

Replace the buffering dependency with the platform's own limiter. A declared
Content-Length over the limit is refused synchronously, before authorization
and before a byte is read. Buffered mode now counts what actually arrives
rather than trusting the header, which closes a gap the previous
implementation had. Streamed mode wraps the body in a counting transform;
on overflow the stream errors, the source is cancelled, and the response is
replaced with the same sanitized 413 even if the handler swallowed the error —
a limit a handler can swallow is not a limit. An unread body is cancelled so
an abandoned upload releases its socket, and an abort stays an ordinary failed
request rather than becoming a 413.

Ordering is unchanged and now strictly better: in streamed mode no byte is
read until the handler reads it, and the handler runs only after authorization
succeeds, so an unauthorized caller cannot push an upload through at all.

The live suite uploads a real 25 MiB body over a real socket end to end.

Co-Authored-By: Claude Opus 5 <[email protected]>

docs/architecture/decisions/0042-bounded-streaming-request-bodies.md

@@ -0,0 +1,160 @@
+# 0042: Explicit, bounded streaming request bodies
+
+- Status: Accepted
+- Date: 2026-08-27
+- Supersedes: none
+- Related: [ADR 0015](0015-static-module-http-routing-and-runtime-lifecycle.md),
+  [ADR 0017](0017-public-request-perimeter-hardening.md),
+  [ADR 0030](0030-host-provided-module-object-storage.md),
+  [ADR 0031](0031-direct-object-delivery.md)
+
+## Context
+
+ADR 0017 gave every public route an enforced request-body ceiling and capped a module route at
+1 MiB, with an explicit note that "large media transfer belongs to a future dedicated storage
+design, never to widening this ceiling."
+
+The storage design arrived — ADR 0030 gave modules a scoped object store, ADR 0031 took Ekklesia out
+of the *read* path with signed delivery URLs — and it did not solve the *write* path. ADR 0031
+deliberately declined to offer signed write URLs, so the only way bytes reach a module's bucket is
+through that module's own HTTP route. That route is capped at 1 MiB, and an ordinary photograph is
+not 1 MiB. A church uploading a slide for a lobby screen cannot, today, get the file in.
+
+Two properties of the existing implementation also need naming, because they are what the change has
+to preserve and what it has to fix:
+
+- The pinned body-limit middleware **fully buffers** any body with no declared `Content-Length`
+  before deciding. At 1 MiB that is a bounded, acceptable cost. At image sizes it would mean the
+  platform holding every concurrent upload in memory.
+- With a declared `Content-Length` within the limit it *counts nothing at all*, trusting the header.
+  Node's HTTP framing makes that safe today, but it is trust the platform does not need to extend.
+
+## Decision
+
+Extend the existing route contract. There is no second HTTP system, no upload endpoint, no signed
+write URL, and no new middleware stack — one route declaration, one limiter, one `413`.
+
+### A route declares how its body reaches it
+
+`ModuleRouteDefinitionInput` gains one optional field:
+
+```ts
+http.route({
+  method: "POST",
+  path: "/uploads",
+  permission: "signage.slides.write",
+  maxBodyBytes: 33_554_432,
+  bodyMode: "streamed",
+  handler,
+});
+```
+
+- **`buffered`** (the default, and what every route written before this meant) — the platform reads
+  and counts the whole body **before** the handler runs, so the handler is promised a fully
+  available, bounded body and an oversized request is rejected before any authorization or handler
+  work happens. Ceiling: 1 MiB, unchanged.
+- **`streamed`** — the platform counts bytes **as the handler consumes them** and never materializes
+  the body. Ceiling: 32 MiB.
+
+The mode is **explicit rather than inferred from the size**. Inferring it — "over 1 MiB means
+streamed" — would mean an author who raised a limit silently changed how their own handler receives
+its body, and when a `413` can arrive relative to their own code. That is exactly the class of
+invisible change this repository refuses.
+
+Omitting the field means `buffered`, so no existing route changed behaviour, and the canonical parsed
+form always carries a mode so nothing downstream re-applies the default or disagrees about it.
+
+### The ceilings, and why 32 MiB
+
+`MODULE_ROUTE_MAX_BODY_BYTES_CEILING` stays at 1 MiB for buffered bodies: a body the platform
+materializes is a body the platform is holding, and 1 MiB is the most a JSON request has any business
+being.
+
+`MODULE_ROUTE_STREAMED_MAX_BODY_BYTES_CEILING` is 32 MiB, sized to hold one 25 MiB image plus bounded
+overhead — a multipart envelope, its boundaries and part headers, and room for a client that pads. It
+is also `MODULE_ROUTE_ABSOLUTE_MAX_BODY_BYTES`: **no route, in any mode, may declare or accept a
+larger single request body, and no unbounded option exists anywhere in the contract.** A route
+declaring more than its mode's ceiling fails registration, exactly as an over-ceiling value always
+has.
+
+The unit is deliberately **one file per request**. A UI offering twenty images issues twenty
+individually bounded requests, each of which the platform can reject cheaply and each of which fails
+alone; a 500 MiB aggregate request would make one bad byte waste everything already sent, and nothing
+in this contract can express one.
+
+### Enforcement, in one limiter
+
+The platform's own limiter replaces the buffering dependency. It is the same code for both modes and
+differs only in *when* it counts:
+
+1. A declared `Content-Length` above the limit is refused synchronously — before authorization,
+   before the handler, before a single body byte is read. That is the cheap rejection an ordinary
+   too-large upload takes. A malformed or contradictory header is treated as *absent* rather than as
+   a rejection, because the byte counting bounds the request either way and refusing could only turn
+   a legitimate request away.
+2. `buffered`: bytes are counted as they arrive and the request is rejected the moment the running
+   total passes the limit — so a lying or absent `Content-Length` is bounded by what actually
+   arrived rather than by what was claimed, which the previous implementation did not do. At most
+   the route's own declared allowance is ever held.
+3. `streamed`: the body is wrapped in a counting transform and handed to the handler. The moment the
+   total passes the limit the stream errors, the source is cancelled, and the response is replaced
+   with the same sanitized `413` — **even if the handler caught the error and answered anyway**. A
+   limit a handler can swallow is not a limit.
+
+A body no handler read is cancelled after the request, so an abandoned upload releases its socket
+rather than draining. An aborted upload surfaces as an ordinary failed request, never a `413`:
+nothing exceeded a limit.
+
+Ordering is unchanged and remains deliberate: trusted-origin check, then the body contract, then
+permission evaluation, then the handler. In `streamed` mode no body byte is read until the handler
+reads it and the handler runs only after authorization succeeds, so an unauthorized caller can never
+push an upload through the platform at all — strictly better than the buffered path.
+
+Timeouts stay where they were: the server's own `headersTimeout`/`requestTimeout`. A slow upload is a
+slow *request*, and the platform already has one answer for that rather than a second per-route one.
+
+### What a module author must know
+
+In `streamed` mode a `413` can arrive **after** the handler has started. A handler that persists as
+it reads must be prepared for a partially consumed upload — write to a temporary key and commit only
+after the read completes. The platform cannot do that for the module, because only the module knows
+what a partial write means to its own data.
+
+## Consequences
+
+- An ordinary image upload works, through the route contract a module already had, with the platform
+  holding none of it.
+- The buffered path now counts bytes it previously trusted a header for, closing a gap rather than
+  opening one.
+- Every existing route and every existing test is unchanged: the default is the old behaviour, and
+  the `413` contract, ordering, and sanitization are identical.
+- `apps/api` owns ~60 lines of stream plumbing it previously delegated. That is the price of a
+  limiter that neither buffers a large body nor can be swallowed by a handler, and it is enforced by
+  tests rather than by reading the dependency's source.
+- Media larger than 32 MiB — a video — still has no path in. That is deliberate and named below.
+
+## Alternatives
+
+- **Raise the single ceiling to 32 MiB.** Rejected: every existing route would silently gain the
+  ability to have 32 MiB buffered in memory on its behalf, and `await request.json()` in a module
+  handler would stop being a bounded operation.
+- **Infer streaming from `maxBodyBytes`.** Rejected: it makes a size change silently change delivery
+  semantics and error timing for the author's own code.
+- **A separate upload endpoint or second HTTP system.** Rejected: a second route table, a second
+  authorization path, a second CORS and origin policy, and a second failure vocabulary, to solve
+  something the existing route contract can express with one field.
+- **Signed write URLs to object storage.** Deliberately still deferred (ADR 0031). A signed write URL
+  is a bearer capability to *create* an object, which needs quota, content-type, and lifecycle
+  answers this pass does not have — and a module that wants to validate, transform, or authorize per
+  byte cannot use one anyway.
+- **Let the handler enforce its own limit.** Rejected: a limit enforced by the party being protected
+  against is not a limit, and every module would reimplement it slightly differently.
+- **Trust `Content-Length` in streamed mode.** Rejected: the running byte count costs nothing and
+  removes the need to reason about framing at all.
+
+## Deferred
+
+Signed write URLs, resumable or chunked multi-request uploads, per-module or per-installation upload
+quotas, content-type or magic-byte validation, virus scanning, image transformation, progress
+reporting, and any body larger than the absolute ceiling — a video pipeline is its own design, not a
+larger number here.

docs/architecture/decisions/README.md

@@ -50,3 +50,4 @@ ADRs capture durable choices that affect more than one package or phase.
 - [0039: Shared design tokens and foundational elements](0039-shared-design-tokens-and-elements.md) — Accepted
 - [0040: Module stylesheets, and every browser artifact gets an isolated root](0040-module-stylesheets-and-artifact-isolation.md) — Accepted
 - [0041: One module may contribute a browser renderer to another module's capability](0041-module-browser-contributions.md) — Accepted
+- [0042: Explicit, bounded streaming request bodies](0042-bounded-streaming-request-bodies.md) — Accepted