feat: support split Dashboard and API origins (ADR 0045)

An installation can now serve the Dashboard and the API on sibling
hostnames (dash./api.) beside the default same-origin topology, from one
image and with configuration alone.

- The Dashboard learns its API origin at run time: the built page carries
  one empty <meta name="ekklesia-api-origin">, which the API fills with its
  identity base URL's origin at startup, before computing the page's ETag.
  The shell accepts only a canonical http(s) origin and otherwise fails
  closed without making a request. The build-time EKKLESIA_API_BASE_URL is
  gone from the bundle; the development server injects the same element.
- The display host stays on the API's origin, so its device cookies,
  requests, and offline cache are unchanged.
- The API trusts its own origin (trusted origins plus the identity base
  URL), as Better Auth already does, so the display page's mutations pass
  the origin check without operators listing it.
- Doctor validates EKKLESIA_DASHBOARD_ORIGIN exactly as the API's startup
  does (it previously skipped it), reports the topology without printing
  hostnames, and warns when split hostnames are not siblings.
- The module UI contract states the CORS header limit (safelisted headers
  plus Content-Type, safelisted response headers only).
- Reference deployment: Caddyfile.split.example (Dashboard hostname serves
  / and its script; API hostname serves /api/*, /display/*, and the
  display script; each refuses the other's paths) and
  EKKLESIA_DASHBOARD_HOSTNAME in the proxy overlay and env examples.
- The deploy smoke test runs either topology (CI runs both). The split run
  installs the module template as a test-only module, checks per-hostname
  routing and exact CORS, then drives a digest-pinned headless Chromium
  over the DevTools protocol, with no added dependency: sign-in on the
  Dashboard hostname, the session cookie host-only on the API hostname,
  the module UI imported cross-origin and its credentialed request
  succeeding, the display and its service worker on the API hostname, and
  sign-out.

ADR 0045 is marked Accepted.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

docs/development/security.md

@@ -153,7 +153,8 @@ enforced by `requirePermission` regardless; the test suite proves it by having a
 "denied" call its API anyway and still receive `403`.
 
 **The host executes trusted module code in its own page and does not pretend otherwise.** The loader
-performs a same-origin dynamic `import()` — no `eval`, no `new Function`, no `blob:` URL — and the
+performs a dynamic `import()` from the API's origin (the page's own in the same-origin topology, the
+API hostname in the split one, ADR 0045) — no `eval`, no `new Function`, no `blob:` URL — and the
 shell owns the container, the loading state, and every error, so a failing module costs its own panel
 rather than the shell. None of that is containment: once imported, a module UI is ordinary same-realm
 JavaScript, exactly as described above.
@@ -493,7 +494,9 @@ outstanding.
    split origin is covered by the exact trusted-origin/CORS configuration. `apps/api` serves the
    built Dashboard itself at `/` ([API § Dashboard host](api.md#dashboard-host)), and the
    reference deployment's proxy routes it ([deploy/README.md](../../deploy/README.md#launch-checklist)
-   maps every item here to its configuration).
+   maps every item here to its configuration). A split installation (ADR 0045) puts its Dashboard
+   and API hostnames on one registrable domain and uses `Caddyfile.split.example`, which refuses
+   `/api/*` on the Dashboard's hostname so the session cookie has exactly one home.
 2. The origin port is unreachable from the public Internet.
 3. The origin port is also unreachable from any untrusted network/VLAN — only the intended private
    proxy path connects. Security must not depend on an unpublished DNS name or origin IP.