Add offline and ntfy alerts

Co-authored-by: Copilot <[email protected]>

modules/app_gps.py

@@ -27,6 +27,7 @@ PWRKEY_HOLD_SECONDS = 4  # matches Waveshare's reference script
 
 LocationX = None
 LocationY = None
+last_fix_at = None
 
 _serial_lock = Lock()
 _serial = None
@@ -148,7 +149,7 @@ def _parse_cgnsinf(resp):
 
 
 def _poll_loop():
-    global LocationX, LocationY
+    global LocationX, LocationY, last_fix_at
     try:
         with _serial_lock:
             if _ensure_module_powered():
@@ -168,11 +169,16 @@ def _poll_loop():
         if fix:
             LocationX = fix["lat"]
             LocationY = fix["lon"]
+            last_fix_at = time.monotonic()
             socketio.emit("gps_location", [LocationX, LocationY])
 
         time.sleep(POLL_INTERVAL)
 
 
+def get_fix_age():
+    return None if last_fix_at is None else time.monotonic() - last_fix_at
+
+
 def start_gps():
     """Start the background thread that continuously polls the SIM868 for a GNSS fix."""
     global _gps_thread