feat: deliver module objects directly to browsers through opaque read URLs

ADR 0030 left exactly one way to get an image to a browser: read the
object into the module's process and write it back out of a module HTTP
route. Every byte of every asset travelled storage -> Ekklesia -> browser.
For the slideshow this platform exists to replace that means Ekklesia in
the path of every image on every display, permanently, for bytes that
never change — and it would make Ekklesia responsible for re-implementing
byte ranges, without which video seeking cannot work at all.

A module can now ask a logical bucket for a temporary browser URL:

    const { url, expiresAt } = await assets.createReadUrl("slides/welcome.png");

The contract names intent, not mechanism. There is no signing, expiry
format, endpoint, or bucket in it — a host with no S3 anywhere could
satisfy the same method, which is what keeps a future single-module
standalone host able to implement it. The URL is opaque to module code and
a bearer capability to whoever holds it, so a module authorizes the
request in its own route before asking: storage bounds reach, never
identity. Lifetimes run one minute to six hours, fifteen by default, and
out-of-range values are refused rather than clamped, so "expires forever"
is unreachable. Logical keys stay canonical state; URLs are generated on
demand and never persisted or logged.

The provider endpoint and the browser delivery origin are separated
because measurement showed they cannot be collapsed: an S3-compatible
signature covers the request Host, so a URL signed for one origin and
rewritten to another is rejected. The browser-facing origin is therefore a
signing input (S3_DELIVERY_ENDPOINT), defaulting to S3_ENDPOINT — so the
standard deployment configures nothing and delivery just works. The scheme
is not signed, so a reverse proxy may terminate TLS provided it passes the
original Host through; that is pinned by a test rather than described and
hoped for.

Read URLs are stable within a window of one third of their lifetime.
Signing is a pure function of the clock, so naive re-signing yields a new
URL per second — a guaranteed browser cache miss, and the reference client
keys its <img> by URL, so an unchanged image would be torn down and
re-downloaded continuously. Quantizing the signing timestamp makes
repeated calls byte-identical while leaving at least two-thirds of the
lifetime; Cache-Control is set at signing time to the guaranteed
remainder, because the pinned SeaweedFS image sends none of its own.

Verified against live SeaweedFS: byte ranges answered directly with 206
and a correct Content-Range, so no range proxy is needed in Core; signing
is local (200 URLs in 66ms), so no batch API is needed; unsigned requests
for the same object are refused, so delivery never depends on a public
bucket; tampered signatures, swapped keys, and swapped buckets all
rejected. Signed write and delete URLs are deliberately not built.

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

core/src/storage/provider.ts

@@ -1,10 +1,11 @@
 import type {
   ModuleStorageBody,
-  ModuleStorageListOptions,
   ModuleStorageListing,
+  ModuleStorageListOptions,
   ModuleStorageObject,
   ModuleStorageObjectMetadata,
   ModuleStoragePutOptions,
+  ModuleStorageReadUrl,
 } from "@ekklesia/contracts";
 
 /**
@@ -42,6 +43,25 @@ export interface StorageProvider {
     options: ModuleStoragePutOptions,
   ): Promise<ModuleStorageObjectMetadata>;
   delete(physicalBucket: string, key: string): Promise<void>;
+  /**
+   * A URL a browser can read one object from directly, valid for `expiresIn` seconds.
+   *
+   * The seconds and the download filename arrive already validated against the contract's bounds —
+   * a provider enforces neither, so two providers can never disagree about what a module is allowed
+   * to ask for.
+   *
+   * A provider that cannot issue such a URL has no business implementing this interface: the
+   * facility is part of the storage contract, not an optional extra a module has to feel around for.
+   * An implementation with no notion of signing could satisfy it by minting its own opaque token.
+   */
+  createReadUrl(
+    physicalBucket: string,
+    key: string,
+    options: {
+      readonly expiresInSeconds: number;
+      readonly downloadFilename?: string | undefined;
+    },
+  ): Promise<ModuleStorageReadUrl>;
   /** A cheap reachability probe for diagnostics only. Its result is **never** persisted and never
    * influences a module's lifecycle state — a provider being briefly unreachable is an operational
    * condition, not an administrative decision (ADR 0030). */