Fix silent watcher thread death with watchdog supervisor

Move time.sleep inside the try/except to catch BaseException-level
failures, and wrap the watcher thread in a supervisor that auto-restarts
it within 5s if it ever exits unexpectedly.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/main.py

@@ -26,11 +26,20 @@ os.makedirs(os.path.join(ANNOTATIONS_DIR, "images"), exist_ok=True)
 os.makedirs(os.path.join(ANNOTATIONS_DIR, "labels"), exist_ok=True)
 
 
+def _watcher_supervisor():
+    while True:
+        t = threading.Thread(target=watcher.run_watcher, daemon=True, name="watcher")
+        t.start()
+        t.join()
+        logging.warning("Watcher thread exited unexpectedly — restarting in 5s")
+        time.sleep(5)
+
+
 @asynccontextmanager
 async def lifespan(app: FastAPI):
     database.init_db()
-    t = threading.Thread(target=watcher.run_watcher, daemon=True)
+    sup = threading.Thread(target=_watcher_supervisor, daemon=True, name="watcher-supervisor")
-    t.start()
+    sup.start()
     yield
 
 

app/watcher.py

@@ -377,6 +377,7 @@ def run_watcher():
                         _last_event_time = now
                         log.info("Vehicle detected!")
                         _process_event()
+            time.sleep(POLL_INTERVAL)
         except Exception as e:
             log.error(f"Watcher loop error: {e}", exc_info=True)
-        time.sleep(POLL_INTERVAL)
+            time.sleep(POLL_INTERVAL)