feat(devices): pairing codes replace the registration broadcast

A pending device registration used to be announced to EVERY account's
Devices page, and whoever clicked Approve first bound the device — and
its freshly minted sync token — to their account. Worse, the pending
card displayed the registration UUID, which is also the bearer
credential on the unauthenticated /api/devices/register/status poll
channel: any family user who could see the list already held everything
needed to take the device. On a family server that is a
wrong-account-binding bug (accidental or deliberate); this commit
removes the class of problem rather than patching the race.

The new model is the Netflix/YouTube TV device flow: the registering
device displays a 6-digit pairing code, and the only person who can
read it — whoever holds the device — types it into THEIR account on the
Devices page. Possession of the device screen becomes the credential;
nothing is broadcast; the reject-on-someone-else's-behalf hole dies
with the list.

Server changes:
- POST /api/devices/register now mints pairing_code (crypto/rand,
  rejection-sampled for uniformity, collision-checked against live
  registrations) and retires prior unclaimed registrations with the
  same device_identifier, so re-registering leaves exactly one live
  attempt.
- POST /api/devices/claim (JWT) binds the registration behind a code to
  the caller's account, reusing the rotate-or-create approval logic
  factored out of ApproveDevice (shared bindRegistrationToUser helper;
  the Android self-approval route keeps working unchanged).
- POST /api/devices/claim/lookup (JWT) returns the confirm-card data
  without binding. Lookup and claim share a per-account budget (12
  attempts/min, DeviceRateLimiter gained a claim case) — the 1M-code
  space over a 5-minute TTL makes guessing infeasible without it.
- POST /api/devices/register/cancel lets the registering device end its
  own attempt (registration UUID bearer, idempotent, never touches a
  claimed registration).
- ListPendingRegistrations / GetPendingRegistrationsData are
  deprecated-by-emptiness: unclaimed registrations return to nobody.
- RejectDevice and its route are deleted — the broadcast list was its
  only consumer, and there is nothing left for anyone to reject.

Additive-only for pinned surfaces: the register response gains
pairing_code (Android DTO untouched), the four KOReader sync surfaces
and approve keep byte-identical shapes. contract_pins.md changelog
records the widening and the reject removal. Docs, schema registry, and
bruno mirrors move together per the drift gate; bruno_gen.go now keeps
hand-authored bruno/podcasts/ (API docs never existed for them, so the
next -update run would have swept them — latent landmine, defused).

Unit tests: code shape, live-collision avoidance, typed-code
normalization, and the resolve scopes (live/expired/claimed/miss).

docs/developer/api/devices/register_device.md

@@ -40,6 +40,7 @@ Register a new device for sync.
   "registration_id": "registration-uuid",
   "auth_url": "https://bookhoard.com/devices/approve/abc123",
   "qr_code": "data:image/png;base64,iVBORw0KG...",
+  "pairing_code": "583912",
   "expires_in": 300,
   "poll_interval": 3,
   "setup_instructions": {
@@ -48,7 +49,14 @@ Register a new device for sync.
 }
 ```
 
-Open `auth_url` (or scan the QR code) while logged in to approve; the registration expires after 5 minutes. Poll `POST /api/devices/register/status` at `poll_interval` seconds until `status` is `approved`, at which point the response includes the device's `auth_token`, `device_id`, and `sync_endpoints`.
+`pairing_code` is the claim credential: the registering device displays it, and the
+only person who can pair the device is whoever reads it (they type it into the
+Devices page of THEIR account — `POST /api/devices/claim`). Unclaimed registrations
+appear in no account's pending list, so there is no first-approver race.
+Registrations expire after 5 minutes; re-registering the same `device_identifier`
+retires the earlier attempt. Poll `POST /api/devices/register/status` at
+`poll_interval` seconds until `status` is `approved`, at which point the response
+includes the device's `auth_token`, `device_id`, and `sync_endpoints`.
 
 ## Error Responses