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]>

surfaces/shell/src/browser/HostApp.tsx

@@ -14,6 +14,7 @@ import {
   HostProtocolError,
   loadHostPrincipal,
 } from "../host-protocol.js";
+import { DevicePanel } from "./DevicePanel.js";
 import { LoginForm } from "./LoginForm.js";
 import { ModuleUiFrame } from "./ModuleUiFrame.js";
 
@@ -157,9 +158,13 @@ async function resolveBootstrapState(
 ): Promise<ResolvedBootstrapState> {
   try {
     const bootstrap = await bootstrapHost(config, signal);
-    return bootstrap.principal.kind === "anonymous"
-      ? { status: "anonymous" }
-      : { bootstrap, status: "ready" };
+    // Only a person signs into the Dashboard. The test is `=== "authenticated"` and never "not
+    // anonymous", so a principal class added later cannot quietly become a Dashboard login — which
+    // is precisely how a paired display device would otherwise have walked in (ADR 0036). In
+    // practice `/api/v1/principal` never reports a device at all; this is the second lock.
+    return bootstrap.principal.kind === "authenticated"
+      ? { bootstrap: { ...bootstrap, principal: bootstrap.principal }, status: "ready" }
+      : { status: "anonymous" };
   } catch (error) {
     if (error instanceof HostProtocolError && error.code === "HOST_UNAUTHENTICATED") {
       return { status: "anonymous" };
@@ -167,9 +172,9 @@ async function resolveBootstrapState(
     if (error instanceof HostProtocolError && error.code === "HOST_ACCESS_DENIED") {
       try {
         const principal = await loadHostPrincipal(config, signal);
-        return principal.kind === "anonymous"
-          ? { status: "anonymous" }
-          : { principal, status: "access-denied" };
+        return principal.kind === "authenticated"
+          ? { principal, status: "access-denied" }
+          : { status: "anonymous" };
       } catch {
         return { status: "failed" };
       }
@@ -454,6 +459,7 @@ export function HostApp(props: HostAppProps): ReactElement {
         </button>
       </header>
       <aside style={shellStyles.aside}>
+        <DevicePanel config={config} />
         {entries.length === 0 && groups.length === 0 ? (
           <p data-testid="host-empty" style={{ color: "#57534e", margin: 0 }}>
             No installed module provides a user interface.