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).

internal/middleware/device_rate_limiter.go

@@ -21,12 +21,14 @@ type DeviceRateLimitConfig struct {
 	SyncRequestsPerMinute     int
 	ProgressUpdatesPerMinute  int
 	MetadataRequestsPerMinute int
+	ClaimAttemptsPerMinute    int
 }
 
 const (
 	DefaultSyncRequestsPerMinute     = 60
 	DefaultProgressUpdatesPerMinute  = 120
 	DefaultMetadataRequestsPerMinute = 30
+	DefaultClaimAttemptsPerMinute    = 12
 	CleanupInterval                  = 5 * time.Minute
 	RequestWindow                    = 1 * time.Minute
 )
@@ -67,12 +69,23 @@ func (drl *DeviceRateLimiter) CheckRateLimit(deviceID string, requestType string
 		limit = config.ProgressUpdatesPerMinute
 	case "metadata":
 		limit = config.MetadataRequestsPerMinute
+	case "claim":
+		limit = config.ClaimAttemptsPerMinute
 	default:
 		limit = DefaultSyncRequestsPerMinute
 	}
 
 	if limit == 0 {
-		limit = DefaultSyncRequestsPerMinute
+		switch requestType {
+		case "progress":
+			limit = DefaultProgressUpdatesPerMinute
+		case "metadata":
+			limit = DefaultMetadataRequestsPerMinute
+		case "claim":
+			limit = DefaultClaimAttemptsPerMinute
+		default:
+			limit = DefaultSyncRequestsPerMinute
+		}
 	}
 
 	info.Requests = append(info.Requests, now)