registration: self-assigned camera IDs (Option B) + tolerate clockless status

Auto-registration never completed: the firmware announced on the wrong
topic, the hub never replied, and an unregistered node couldn't receive a
reply anyway. Switch to self-assigned IDs:

firmware (esp32-mqtt-bridge.cpp):
- camera_id defaults to the device id (clientID, e.g. rig-86d978)
- always subscribe to <id>/command; announce on the contract topic
  remoterig/cameras/<id>/announce (was the unmatched announce-<id> form)
- drop the bogus numeric timestamp from status (node has no clock)

hub (subscriber.go):
- handleAnnounce registers new cameras under the node's self-assigned id
  (no cam-NNN, no registered reply)
- handleStatus tolerates an empty/invalid timestamp and stamps server-side
  (previously rejected the status outright)

docs/MQTT_CONTRACT.md updated to match.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

internal/mqtt/subscriber.go

@@ -151,22 +151,20 @@ func (s *Subscriber) handleStatus(cameraID string, payload []byte) {
 	}
 
 	// Validate required fields
-	if sp.CameraID == "" || sp.Timestamp == "" {
-		log.Printf("MQTT status missing required fields (camera_id, timestamp) from %s", cameraID)
+	if sp.CameraID == "" {
+		log.Printf("MQTT status missing camera_id from %s", cameraID)
 		return
 	}
 
-	// Validate timestamp sanity (reject >5min future, >24h past)
+	// Nodes have no real clock, so tolerate an empty/invalid timestamp by
+	// stamping server-side. Still clamp obviously-bad supplied times below.
+	now := time.Now()
 	ts, err := time.Parse(time.RFC3339, sp.Timestamp)
 	if err != nil {
-		// Try ISO8601 without timezone
-		ts, err = time.Parse("2006-01-02T15:04:05", sp.Timestamp)
-		if err != nil {
-			log.Printf("MQTT status invalid timestamp %q from %s", sp.Timestamp, cameraID)
-			return
+		if ts, err = time.Parse("2006-01-02T15:04:05", sp.Timestamp); err != nil {
+			ts = now
 		}
 	}
-	now := time.Now()
 	if ts.After(now.Add(5 * time.Minute)) {
 		log.Printf("MQTT status timestamp too far in future (%s) from %s — using now", ts, cameraID)
 		ts = now
@@ -374,30 +372,20 @@ func (s *Subscriber) handleAnnounce(cameraID string, payload []byte) {
 		}
 		log.Printf("MQTT announce: camera %s (%s) re-connected", existingID, ap.FriendlyName)
 	} else {
-		// New camera — generate sequential cam-NNN ID
-		var maxID string
-		s.db.QueryRow("SELECT MAX(camera_id) FROM cameras").Scan(&maxID)
-
-		seq := 1
-		if maxID != "" {
-			fmt.Sscanf(maxID, "cam-%d", &seq)
-			seq++
-		}
-
-		newID := fmt.Sprintf("cam-%03d", seq)
+		// Option B: the node self-assigns its camera_id (the announce topic id).
 		_, err = s.db.Exec(`
 			INSERT INTO cameras (camera_id, friendly_name, mac_address, created_at, updated_at)
 			VALUES (?, ?, ?, datetime('now'), datetime('now'))
-		`, newID, ap.FriendlyName, ap.MacAddress)
+		`, cameraID, ap.FriendlyName, ap.MacAddress)
 		if err != nil {
 			log.Printf("MQTT announce insert error for %s: %v", ap.MacAddress, err)
 			return
 		}
 
-		log.Printf("MQTT announce: new camera registered as %s (%s)", newID, ap.FriendlyName)
+		log.Printf("MQTT announce: new camera registered as %s (%s)", cameraID, ap.FriendlyName)
 
 		// Broadcast new camera via SSE
-		cam, err := getCamera(s.db, newID)
+		cam, err := getCamera(s.db, cameraID)
 		if err == nil {
 			s.hub.Broadcast("camera_registered", cam)
 		}