Add events/rules engine, sensor bridge, and UPS monitor (Phases 6-8)

Phase 6 — Events + Rule Engine:
- EventProcessor subprocess: subscribes to all MQTT events, logs to DB,
  evaluates rules, fires alert actions
- ArmStateFSM: DISARMED/ARMED_HOME/ARMED_AWAY with PIN verification
  (HMAC-safe), DB persistence, MQTT state publishing
- RuleEngine: AND/OR logic, 4 condition types (arm_state, sensor_event,
  camera_motion, time_window), per-rule cooldown tracking
- SSE event stream with subscriber queue pattern and keepalive
- Event acknowledge endpoint

Phase 7 — Sensor Bridge:
- SensorBridge subprocess: subscribes to Zigbee2MQTT, normalizes payloads
  (contact, occupancy, temperature, humidity, battery, linkquality)
- GPIOHandler: conditional gpiozero import, callbacks for reed switches
  and PIR sensors
- SensorRegistry: maps Zigbee addresses and names to config sensor IDs
- SensorEvent/SensorState dataclasses
- Web UI now shows real sensor states from DB

Phase 8 — UPS Monitor:
- UPSMonitor subprocess: polls NUT via pynut2 with reconnect backoff
- State transition detection: OL→OB (power_loss), charge/runtime
  thresholds (low_battery, critical), OB→OL (restored)
- ShutdownSequence: ordered shutdown with configurable delay and command
- All conditionally imported (pynut2, gpiozero) for non-target platforms

Fixed test_db fixture to use isolated engines (no global singleton leak).
96 tests passing.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tests/unit/test_schema.py

@@ -1,15 +1,17 @@
 """Tests for database schema creation."""
 
-from vigilar.storage.db import init_db
+from sqlalchemy import create_engine, inspect
+
 from vigilar.storage.schema import metadata
 
 
 def test_tables_created(tmp_path):
     db_path = tmp_path / "test.db"
-    engine = init_db(db_path)
+    engine = create_engine(f"sqlite:///{db_path}", echo=False)
+    metadata.create_all(engine)
+
     assert db_path.exists()
 
-    from sqlalchemy import inspect
     inspector = inspect(engine)
     table_names = inspector.get_table_names()