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>

firmware/src/esp32-mqtt-bridge.cpp

@@ -311,23 +311,25 @@ bool connectMQTT() {
 
   Serial.println("[MQTT] Connected");
 
-  // Subscribe to commands (if registered)
-  if (cfg.camera_id.length() > 0) {
-    mqtt.subscribe(mqttTopic("command").c_str(), 2);
+  // Option B: self-assigned, stable camera_id derived from the device id.
+  if (cfg.camera_id.length() == 0) {
+    cfg.camera_id = clientID();   // e.g. "rig-86d978"
   }
 
-  // Announce if new
-  if (cfg.camera_id.length() == 0) {
+  // Subscribe to our command topic.
+  mqtt.subscribe(mqttTopic("command").c_str(), 2);
+
+  // Announce (retained) on the contract topic so the hub registers/tracks us.
+  {
     JsonDocument doc;
     doc["mac_address"]      = WiFi.macAddress();
-    doc["firmware_version"] = "0.3.0-esp32-mqtt-bridge";
-    doc["friendly_name"]    = "Cam-" + clientID();
+    doc["firmware_version"] = "0.4.0-esp32-mqtt-bridge";
+    doc["friendly_name"]    = "Cam-" + cfg.camera_id;
     JsonArray caps = doc["capabilities"].to<JsonArray>();
     caps.add("start_stop"); caps.add("status");
     String payload; serializeJson(doc, payload);
-    String announceTopic = "remoterig/cameras/announce-" + clientID();
-    mqtt.publish(announceTopic.c_str(), payload.c_str(), true);
-    Serial.println("[MQTT] Announced for registration");
+    mqtt.publish(mqttTopic("announce").c_str(), payload.c_str(), true);
+    Serial.printf("[MQTT] Announced as %s\n", cfg.camera_id.c_str());
   }
 
   return true;
@@ -489,7 +491,7 @@ void loop() {
         // Build the MQTT status payload per contract
         JsonDocument mqttDoc;
         mqttDoc["camera_id"]  = cfg.camera_id;
-        mqttDoc["timestamp"]  = millis();
+        // No timestamp: the node has no real clock; the hub stamps on receipt.
         mqttDoc["battery_raw"] = dispBatteryRaw;
         int pct = batteryPct(dispBatteryRaw);
         if (pct >= 0) mqttDoc["battery_pct"] = pct;   // omit when uncalibrated