feat: SQLite persistence, non-blocking inference, modular frontend

Backend:
- Persist alert events and robot delivery counters in SQLite (data/app.db,
  YOLO_DB_PATH override); EventStore keeps its interface, DeliveryStatsStore
  uses read/write-through UPSERT; in-memory fallback remains for standalone
  AlertManager use
- Serve /api/detect from a sync endpoint and serialize YOLO inference with a
  lock, so concurrent requests no longer block the event loop
- Unify single-image and session detection: requests without session_id
  share the fixed single-image session and run full frame confirmation
- Store UTC Z-suffixed timestamps for sortable string comparison; close
  executor and database on shutdown via FastAPI lifespan

Frontend:
- Split app.js into ES modules (main.js + modules/{dom,api,ui,charts,store}
  + views/{dashboard,inspection,events,robots,robotCards}), no build step
- Escape all server data interpolated into innerHTML; guard missing
  event.classes; replace lazy element ID list with memoized qs()
- Remove hardcoded fake stats (trend badge, device donut segment)

Docs:
- Rewrite README: accurate weight policy (only trained best.pt committed,
  no Git LFS), SQLite persistence, single-worker note, data asset inventory
- Align models/pretrained/README.md with actual files; add YOLO_DB_PATH to
  .env.example; add persistence unit tests; ruff clean

test/test_alerting.py

@@ -1,9 +1,12 @@
 from __future__ import annotations
 
+import tempfile
 import unittest
+from pathlib import Path
 from unittest.mock import patch
 
 from backend.alerting import AlertManager
+from backend.storage import Database, DeliveryStatsStore
 
 
 class AlertManagerRobotTests(unittest.TestCase):
@@ -51,6 +54,28 @@ class AlertManagerRobotTests(unittest.TestCase):
         with self.assertRaisesRegex(ValueError, "Unsupported"):
             manager.test_channel("unknown")
 
+    def test_delivery_stats_persist_across_store_instances(self) -> None:
+        with tempfile.TemporaryDirectory() as tmp_dir:
+            db_path = Path(tmp_dir) / "app.db"
+            first_db = Database(db_path)
+            store = DeliveryStatsStore(first_db)
+            store.record("wechat", success=True)
+            store.record("wechat", success=False, error="boom")
+            store.record("feishu", success=True, test=True)
+            first_db.close()
+
+            second_db = Database(db_path)
+            reloaded = DeliveryStatsStore(second_db)
+            stats = reloaded.get_all()
+            second_db.close()
+
+        self.assertEqual(stats["wechat"]["delivered"], 1)
+        self.assertEqual(stats["wechat"]["failed"], 1)
+        self.assertEqual(stats["wechat"]["last_status"], "failed")
+        self.assertEqual(stats["wechat"]["last_error"], "boom")
+        self.assertEqual(stats["feishu"]["tests"], 1)
+        self.assertIsNotNone(stats["feishu"]["last_test_at"])
+
 
 if __name__ == "__main__":
     unittest.main()