feat: give display devices their own paired identity

ADR 0033 gave Ekklesia a display host and ADR 0034 gave it a login page.
Together, the only way to light a television in a foyer was to sign a
human being into it and leave that session running on a screen anyone can
walk up to: a person's credentials on a wall, a person's session as the
thing signage depends on, and the display going dark when they are
revoked or their session simply expires.

A screen is not a person. Ekklesia now recognizes three principal classes
— anonymous, authenticated human, and managed device — and a device is a
first-class principal with its own credential and its own explicit
permissions. It has no email, no password, and no row in the identity
tables, and it satisfies the `authenticated` baseline never, so every
route that settles for "somebody is signed in" stays closed to it.

Device permissions live in their own table because
ekklesia_core_grant.user_id is a non-null foreign key into the user table
and making it polymorphic would rewrite the platform's central
authorization record to serve one narrow case. That is a schema decision,
not a second decision point: the evaluator gained one branch, so a device
is allowed or denied by the same function, with the same deny-by-default,
as anyone else — with no owner path it could reach and no baseline that
stands in for a grant.

The six-digit code is not the credential. It is printed across a screen in
a public hallway, so it is treated as public: it identifies a request to
an administrator and is worth nothing alone. Collecting the credential
also requires a 256-bit claim secret held only in the requesting browser's
HttpOnly cookie, and approving requires the restricted core.devices.manage
permission. Without that split, reading a code off a wall would hand over
the screen's identity.

The permission a pairing asks for is read from the module's validated
display declaration, never from the request, so a browser cannot ask to be
granted core.devices.manage or another module's permission. Approval
grants exactly that one permission.

Credentials are stored only as SHA-256; the raw value exists in the
Set-Cookie that issues it and the device's own cookie jar, and is never
logged, returned in JSON, or placed in a URL. The cookie is HttpOnly, so
the module display code running in that same page cannot read it.
Revocation is a server-side column checked on every principal resolution,
and re-pairing mints a new identity so an old credential can never come
back. State transitions are conditional updates, so one code cannot be
approved twice or claimed into two credentials under a race.

/api/v1/principal still reports a device as anonymous: it is what the
Dashboard uses to decide whether a person is signed in. Displays read
their viewer from the descriptor instead, which already performed the
authorization — so displays are descriptor-first while the Dashboard stays
principal-first, and neither infers an identity by elimination.

Proven end to end against real PostgreSQL and real HTTP: an unpaired
screen shows a code, an owner approves it, the device opens the display
descriptor and a realtime stream, and revocation closes both while the
device still holds its cookie. An already-open SSE stream is bounded by
realtime's existing five-minute reconnect rather than closed instantly,
and that bound is documented rather than called immediate.

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

apps/api/tests/module-display-routes.test.ts

@@ -220,7 +220,7 @@ function signedIn(): EkklesiaPrincipal {
 }
 
 describe("module display routes — authorization and descriptor", () => {
-  it("does not let an anonymous viewer open either the page or descriptor", async () => {
+  it("gives an unpaired viewer the inert shell but never the descriptor", async () => {
     const { app } = await install({ display: true });
 
     const [page, descriptor] = await Promise.all([
@@ -228,18 +228,22 @@ describe("module display routes — authorization and descriptor", () => {
       app.request(moduleDisplayDescriptorUrl("announcements")),
     ]);
 
-    expect(page.status).toBe(403);
+    // The page is deliberately open: a television nobody has paired has no identity to present, and
+    // a 403 here would leave it nowhere to show a pairing code (ADR 0036). It carries nothing —
+    // the same module-agnostic bytes every display gets, with no module id and no data.
+    expect(page.status).toBe(200);
+    expect(await page.text()).toBe(displayHostAssets.html);
+
+    // Authorization is entirely in the descriptor, and it has not moved.
     expect(descriptor.status).toBe(403);
-    expect(page.headers.get("cache-control")).toBe("no-store");
     expect(descriptor.headers.get("cache-control")).toBe("no-store");
-    expect(await page.json()).toEqual({ code: "AUTHORIZATION_DENIED" });
     expect(await descriptor.json()).toEqual({ code: "AUTHORIZATION_DENIED" });
   });
 
   it("requires the declared display permission, not merely an authenticated principal", async () => {
     const { app } = await install({ display: true, principal: signedIn() });
 
-    expect((await app.request(moduleDisplayPageUrl("announcements"))).status).toBe(403);
+    // Being signed in is not being permitted, and the descriptor is where that is decided.
     expect((await app.request(moduleDisplayDescriptorUrl("announcements"))).status).toBe(403);
   });
 
@@ -260,6 +264,7 @@ describe("module display routes — authorization and descriptor", () => {
     expect(page.headers.get("x-content-type-options")).toBe("nosniff");
     expect(await page.text()).toBe(displayHostAssets.html);
     expect(await descriptor.json()).toEqual({
+      viewer: { kind: "authenticated", userId: viewerId },
       module: {
         api: { baseUrl: moduleApiBaseUrl("announcements") },
         display: {