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]>

apps/api/src/config.ts

@@ -200,3 +200,19 @@ export function createApiRuntimeConfig(
     storage,
   });
 }
+
+/**
+ * The exact browser origins the API trusts: CORS on `/api/auth/*` and `/api/v1/*`, and every
+ * mutation route's origin check (ADR 0045).
+ *
+ * The configured trusted origins plus the identity base URL's own origin, which is where the API
+ * lives. Better Auth already trusts its base URL implicitly; this makes the API's own checks agree,
+ * so a page the API itself serves (the display host, and the Dashboard in the same-origin topology)
+ * passes without an operator listing the API's origin. Order is preserved and duplicates removed;
+ * every entry is already a canonical origin.
+ */
+export function apiTrustedOrigins(
+  identity: Pick<IdentityConfig, "baseUrl" | "trustedOrigins">,
+): readonly string[] {
+  return Object.freeze([...new Set([...identity.trustedOrigins, identity.baseUrl])]);
+}

apps/api/src/dashboard-host-assets.ts

@@ -54,15 +54,54 @@ export function createDashboardHostAssets(html: string, script: string): Dashboa
   });
 }
 
-/** Loads only the two surface-declared files. A missing or unreadable build fails startup rather
- * than leaving an API whose own origin answers `/` with a 404. */
-export async function loadDashboardHostAssets(): Promise<DashboardHostAssets> {
+/**
+ * The exact element the built Dashboard page carries for its API origin (ADR 0045), empty. The shell
+ * reads the filled-in value at run time; empty means the page's own origin.
+ */
+export const DASHBOARD_API_ORIGIN_PLACEHOLDER = '<meta content="" name="ekklesia-api-origin" />';
+
+function isCanonicalHttpOrigin(value: string): boolean {
+  try {
+    const url = new URL(value);
+    return (url.protocol === "https:" || url.protocol === "http:") && url.origin === value;
+  } catch {
+    return false;
+  }
+}
+
+/**
+ * Writes the API's own origin into the Dashboard page, once, before the page's `ETag` is computed.
+ *
+ * The origin is the identity base URL's, which is where `/api` lives in either topology: the page's
+ * own origin when the Dashboard and the API share one, the API hostname when they are split. The
+ * built page must carry exactly one empty placeholder; anything else is a shell build this API does
+ * not understand, and startup fails rather than serving a page that might send credentials somewhere
+ * unintended. The value is escaped for an attribute even though a canonical origin cannot contain
+ * any character that needs it.
+ */
+export function renderDashboardPage(html: string, apiOrigin: string): string {
+  const parts = html.split(DASHBOARD_API_ORIGIN_PLACEHOLDER);
+  if (parts.length !== 2 || !isCanonicalHttpOrigin(apiOrigin)) {
+    throw new DashboardHostAssetError();
+  }
+  const escaped = apiOrigin
+    .replaceAll("&", "&amp;")
+    .replaceAll('"', "&quot;")
+    .replaceAll("<", "&lt;")
+    .replaceAll(">", "&gt;");
+  return parts.join(`<meta content="${escaped}" name="ekklesia-api-origin" />`);
+}
+
+/** Loads only the two surface-declared files, and renders the API origin into the page. A missing,
+ * unreadable, or incompatible build fails startup rather than leaving an API whose own origin answers
+ * `/` with a 404 or a page that cannot find its API. */
+export async function loadDashboardHostAssets(apiOrigin: string): Promise<DashboardHostAssets> {
   try {
     // Sequential deliberately, as for the display host: diagnostics stay deterministic and no
     // rejected sibling read is left running after this function has already failed.
     const html = await readFile(new URL(dashboardHostAssetUrls.html), "utf8");
     const script = await readFile(new URL(dashboardHostAssetUrls.script), "utf8");
-    return createDashboardHostAssets(html, script);
+    return createDashboardHostAssets(renderDashboardPage(html, apiOrigin), script);
   } catch {
     throw new DashboardHostAssetError();
   }

apps/api/src/server.ts

@@ -44,7 +44,7 @@ import {
   createClientAddressResolver,
   wrapFetchWithClientAddress,
 } from "./client-address.js";
-import { type ApiRuntimeConfig, createApiRuntimeConfig } from "./config.js";
+import { type ApiRuntimeConfig, apiTrustedOrigins, createApiRuntimeConfig } from "./config.js";
 import { loadDashboardHostAssets } from "./dashboard-host-assets.js";
 import { type ApiDatabaseHandle, createApiDatabase } from "./database.js";
 import { withDevicePrincipals } from "./device-principal.js";
@@ -550,7 +550,9 @@ export async function startApi(
       moduleUi.listDisplays().length === 0 ? undefined : await loadDisplayHostAssets();
     // Unconditional, unlike the display host: every installation serves its Dashboard (ADR 0044),
     // so a missing shell build fails here, before any module code is imported.
-    dashboardHostAssets = await loadDashboardHostAssets();
+    // The page learns its API origin from the API that serves it, so one build serves either
+    // topology (ADR 0045).
+    dashboardHostAssets = await loadDashboardHostAssets(config.identity.baseUrl);
   } catch (error) {
     await safeCloseStorage(storageProvider);
     await safeCloseMail(mailProvider);
@@ -648,7 +650,8 @@ export async function startApi(
       // Principal resolution, extended with managed devices. A human session still wins, and this
       // is the only place a device credential ever becomes a principal.
       runtime: withDevicePrincipals({ devices: deviceStore, runtime }),
-      trustedOrigins: config.identity.trustedOrigins,
+      // The configured origins plus the API's own (ADR 0045).
+      trustedOrigins: apiTrustedOrigins(config.identity),
     });
   } catch (error) {
     await safeStopModuleRuntime(moduleRuntime);