feat: device pairing, telemetry ingestion, live dashboard WS (Workstream G)

Implements the backend half of the ESP32-P4 sensor node spec's pairing,
ingestion, and live-broadcast contract:

- New Device model (backend/app/models/device.py): id, user_id FK, name,
  token_hash (unique+indexed), created_at, last_seen_at. Reuses
  generate_session_token()/hash_token() from auth_session.py verbatim for
  the one-time raw pairing token / stored hash.
- POST /api/device, GET /api/device (session-cookie authenticated REST
  pairing endpoints) and POST /api/device/telemetry (device bearer-token
  authenticated ingestion, per-device rate limited, 16KB body cap, 64
  reading cap, strict shape validation — never a 500 on garbage input) in
  backend/app/routes/device.py.
- /ws/device-feed live dashboard WS (qm_session cookie authenticated),
  fanning out ingested readings to the owning user's connected dashboard
  sockets via an in-process dict[user_id, connections] registry, each with
  its own send-queue + single sender task (mirrors app.ws's
  SeanceState/_sender convention).
- last_seen_at updates on every successful ingestion.
- _process_reading(device, reading) left as an explicit no-op handoff point
  for Workstream K's summon-pipeline integration.

Backend suite: 102 passed.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/tests/conftest.py

@@ -49,9 +49,11 @@ def sync_client(monkeypatch):
     swapped: TestClient runs the lifespan on its own portal loop, and the
     pooled production engine would carry connections across loops."""
     import app.main as main_module
+    import app.routes.device as device_module
     import app.ws as ws_module
 
     monkeypatch.setattr(ws_module, "session_maker", TestSessionLocal)
+    monkeypatch.setattr(device_module, "session_maker", TestSessionLocal)
     monkeypatch.setattr(main_module, "engine", test_engine)
     with TestClient(app, base_url="https://testserver") as tc:
         yield tc