Add monitor API tests (B8)

- test_monitor.py: 12 test cases covering:
  - API key generation (success, 404, admin-only)
  - heartbeat-v2 endpoint (valid key, invalid key, missing key)
  - API key revocation and validation
  - ServerState data persistence (agents_json, cpu_pct, etc.)
  - Disabled server rejection
- conftest.py: import app.models.monitor for test database setup

All tests passing (pytest -v).

tests/conftest.py

@@ -32,6 +32,10 @@ try:
     import app.models.webhook  # noqa: F401
 except ImportError:
     pass
+try:
+    import app.models.monitor  # noqa: F401
+except ImportError:
+    pass
 
 TEST_DATABASE_URL = "sqlite://"  # in-memory