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/api-reference.md

@@ -144,9 +144,11 @@ See [Device Registration & Sync](devices/get_devices.md)
 - GET /api/devices/:id - Get device
 - PUT /api/devices/:id - Update device
 - DELETE /api/devices/:id - Delete device
-- GET /api/devices/pending - List pending registrations (admin)
-- GET /api/devices/approve/:registration_id - Approve registration (admin)
-- POST /api/devices/reject/:registration_id - Reject registration (admin)
+- GET /api/devices/pending - List pending registrations (always empty — pairing codes replaced the broadcast list)
+- GET /api/devices/approve/:registration_id - Approve registration by UUID (Android self-approval path)
+- POST /api/devices/claim - Claim a registration by its pairing code ([Pairing codes](devices/claim_device.md))
+- POST /api/devices/claim/lookup - Resolve a pairing code without binding ([Lookup](devices/lookup_pairing_code.md))
+- POST /api/devices/register/cancel - The registering device cancels its own pairing attempt ([Cancel](devices/cancel_device_registration.md))
 - POST /api/devices/:id/shelves - Add to shelf (Kobo; used by native Kobo sync, coming soon)
 - GET /api/devices/:id/shelves - Get shelf contents
 - DELETE /api/devices/:id/shelves - Remove from shelf