hub: fix camera listing, heartbeat parse, and legacy-id migration

Three bugs surfaced once the camera reported in:

- ListCameras LEFT JOIN returns NULL status columns for a camera with no
  status rows yet, which failed scanning into non-nullable int/time fields
  (recording_state, online, recorded_at) and emptied the whole list.
  COALESCE them (recorded_at falls back to the camera's created_at).
- handleHeartbeat rejected every heartbeat ("cannot unmarshal number into
  string") because the node sends a numeric millis() timestamp. The handler
  doesn't use it, so drop the Timestamp field and let it be ignored.
- handleAnnounce kept a stale cam-NNN row registered by MAC under the old
  (pre-self-id) scheme, so self-id status inserts hit a FOREIGN KEY error.
  When a MAC is known under a different id than the node's self-id, migrate:
  drop the old row and re-register under the self-id.

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

internal/api/cameras.go

@@ -26,12 +26,12 @@ func ListCameras(database *db.DB) http.HandlerFunc {
 				c.friendly_name,
 				s.battery_pct,
 				s.video_remaining_sec,
-				s.recording_state,
+				COALESCE(s.recording_state, 0),
 				s.mode,
 				s.resolution,
 				s.fps,
-				s.online,
-				s.recorded_at
+				COALESCE(s.online, 0),
+				COALESCE(s.recorded_at, c.created_at)
 			FROM cameras c
 			LEFT JOIN (
 				SELECT camera_id, battery_pct, video_remaining_sec, recording_state,