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]>

docs/development/authorization.md

@@ -275,6 +275,30 @@ unregistered target permission instead receives the identical sanitized
 `403 {"code":"AUTHORIZATION_DENIED"}` a genuinely denied caller would, so that response never
 reveals whether the caller was denied or the permission was simply never registered.
 
+## Device permissions
+
+A paired display device holds explicit permissions of its own, in `ekklesia_core_device_permission`
+rather than in `ekklesia_core_grant`
+([ADR 0036](../architecture/decisions/0036-managed-device-identity-and-pairing.md)). That table's
+`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.
+
+**It is a separate table, not a separate decision.** `createAuthorizationEvaluator` consults exactly
+one authority per call, chosen by principal kind, and a device reaches `allowed` only by holding the
+exact permission being checked:
+
+- `public` — allowed, as for any principal.
+- `authenticated` — **denied**, always. A device is not a person.
+- `restricted` — allowed only with an active, unrevoked permission for that exact device.
+
+A device is never a platform owner, and the owner and user-grant ports are not consulted for one at
+all. A deployment that composed no device support denies every device. Revoking a device revokes its
+permissions with it, and the permission lookup joins the device row so a revoked device has no
+permissions even if a row were left behind.
+
+Approving a pairing grants exactly the one display permission the host derived from the module's
+validated display declaration — never a bundle, never a permission the browser named.
+
 ## Platform-owner bootstrap
 
 There is no "first user becomes owner" inference anywhere in this repository, and no HTTP