Add processor heartbeat management and improve health check functionality

- Introduced processor heartbeat configuration in environment variables and Docker setup.
- Implemented heartbeat publishing in the processor worker.
- Enhanced health check endpoint to include processor availability status.
- Updated frontend components to handle processor unavailability messages.
- Added legacy return functionality in the upload panel.

faceai/apps/backend/src/redis-store.js

@@ -24,6 +24,10 @@ function rateLimitKey(userId) {
   return `faceai:rate-limit:${userId}`;
 }
 
+function processorHeartbeatKey() {
+  return 'faceai:processor-heartbeat';
+}
+
 export async function incrementRateLimit(redis, userId, windowSeconds) {
   const key = rateLimitKey(userId);
   const count = await redis.incr(key);
@@ -50,6 +54,21 @@ export async function getActiveSearchId(redis, userId) {
   return redis.get(activeSearchKey(userId));
 }
 
+export async function updateProcessorHeartbeat(redis, ttlSeconds, payload = {}) {
+  const heartbeat = {
+    updatedAt: Date.now(),
+    ...payload
+  };
+
+  await redis.set(processorHeartbeatKey(), JSON.stringify(heartbeat), 'EX', ttlSeconds);
+  return heartbeat;
+}
+
+export async function getProcessorHeartbeat(redis) {
+  const raw = await redis.get(processorHeartbeatKey());
+  return raw ? JSON.parse(raw) : null;
+}
+
 export async function createSearchRecord(redis, payload, ttlSeconds) {
   const searchId = randomId('search');
   const record = {