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/cancel_device_registration.md

@@ -0,0 +1,42 @@
+---
+method: POST
+path: /api/devices/register/cancel
+auth: none
+area: devices
+---
+<!-- source: internal/handlers/devices.go DeviceHandler.CancelRegistration -->
+
+# Cancel Device Registration
+
+Let the REGISTERING device end its own pairing attempt — the plugin calls this
+when the user dismisses its waiting dialog. The `registration_id` is the
+bearer credential (same trust as the status poll): a device can only ever
+cancel its own registration, and already-claimed registrations are left alone
+so their token delivery to the device is never disrupted. Idempotent: the
+response is the same whether or not the registration still exists.
+
+**Endpoint**: `POST /api/devices/register/cancel`
+**Auth**: Not required (registration UUID bearer, like the status poll)
+**Content-Type**: `application/json`
+
+## Request Body
+
+| Field           | Type   | Required | Description                    |
+| --------------- | ------ | -------- | ------------------------------ |
+| registration_id | string | Yes      | UUID from the register response |
+
+### Example Request
+
+```json example-request
+{
+  "registration_id": "550e8400-e29b-41d4-a716-446655440000"
+}
+```
+
+## Response (200 OK)
+
+```json example-response
+{
+  "message": "registration cancelled"
+}
+```