feat(events): processor handles SYSTEM_ARM_REQUEST over MQTT

Adds _handle_arm_request and a dedicated bus.subscribe on
Topics.SYSTEM_ARM_REQUEST. Payload {mode, pin, triggered_by} is
dispatched to ArmStateFSM.transition, which verifies the PIN via
alerts.pin.verify_pin and performs the state change.

This is the missing link for web /system/api/arm to actually move
the system into an armed state. Part of issue #2.

tests/unit/test_events.py

@@ -511,3 +511,54 @@ class TestEventsPublishedBroadcast:
         )
 
         assert not any(t == Topics.EVENTS_PUBLISHED for t, _ in bus.published)
+
+
+# ---------------------------------------------------------------------------
+# Arm Request Dispatch
+# ---------------------------------------------------------------------------
+
+class TestArmRequestDispatch:
+    """SYSTEM_ARM_REQUEST messages must reach ArmStateFSM.transition."""
+
+    def test_arm_request_calls_fsm_transition(self, test_db):
+        from vigilar.events.processor import EventProcessor
+
+        processor = EventProcessor.__new__(EventProcessor)
+
+        calls = []
+
+        class FakeFSM:
+            state = ArmState.DISARMED
+
+            def transition(self, new_state, pin="", triggered_by="system"):
+                calls.append((new_state, pin, triggered_by))
+                return True
+
+        processor._handle_arm_request(
+            payload={"mode": "ARMED_AWAY", "pin": "1234", "triggered_by": "web"},
+            fsm=FakeFSM(),
+        )
+
+        assert len(calls) == 1
+        new_state, pin, triggered_by = calls[0]
+        assert new_state == ArmState.ARMED_AWAY
+        assert pin == "1234"
+        assert triggered_by == "web"
+
+    def test_arm_request_ignores_bad_mode(self, test_db):
+        from vigilar.events.processor import EventProcessor
+
+        processor = EventProcessor.__new__(EventProcessor)
+        calls = []
+
+        class FakeFSM:
+            def transition(self, *a, **kw):
+                calls.append((a, kw))
+                return True
+
+        processor._handle_arm_request(
+            payload={"mode": "NONSENSE", "pin": "1234"},
+            fsm=FakeFSM(),
+        )
+
+        assert calls == []

vigilar/events/processor.py

@@ -9,7 +9,7 @@ from sqlalchemy.engine import Engine
 
 from vigilar.bus import MessageBus
 from vigilar.config import VigilarConfig
-from vigilar.constants import EventType, Severity, Topics
+from vigilar.constants import ArmState, EventType, Severity, Topics
 from vigilar.events.rules import RuleEngine
 from vigilar.events.state import ArmStateFSM
 from vigilar.storage.db import get_db_path, init_db
@@ -58,12 +58,20 @@ class EventProcessor:
         fsm.set_bus(bus)
         bus.connect()
 
-        # Subscribe to all Vigilar topics
+        # Subscribe to all Vigilar topics (events/motion/sensors/etc.)
         def on_message(topic: str, payload: dict[str, Any]) -> None:
             self._handle_event(topic, payload, engine, fsm, rule_engine, bus)
 
         bus.subscribe_all(on_message)
 
+        # Dedicated subscription for web-originated arm/disarm requests.
+        # Kept separate from on_message because these are commands, not
+        # classifiable events.
+        def on_arm_request(topic: str, payload: dict[str, Any]) -> None:
+            self._handle_arm_request(payload, fsm)
+
+        bus.subscribe(Topics.SYSTEM_ARM_REQUEST, on_arm_request)
+
         log.info("Event processor started")
 
         # Main loop
@@ -160,6 +168,26 @@ class EventProcessor:
         except Exception:
             log.exception("Error processing event on %s", topic)
 
+    def _handle_arm_request(
+        self,
+        payload: dict[str, Any],
+        fsm: "ArmStateFSM",
+    ) -> None:
+        """Handle an arm/disarm request received over MQTT.
+
+        Payload fields:
+          - mode: str — desired ArmState ("DISARMED", "ARMED_HOME", "ARMED_AWAY")
+          - pin: str — plaintext PIN (FSM verifies against security.pin_hash)
+          - triggered_by: str — origin tag for the audit log (e.g. "web")
+        """
+        mode = payload.get("mode", "")
+        if mode not in ArmState.__members__:
+            log.warning("Ignoring arm request with invalid mode: %r", mode)
+            return
+        pin = payload.get("pin", "")
+        triggered_by = payload.get("triggered_by", "unknown")
+        fsm.transition(ArmState(mode), pin=pin, triggered_by=triggered_by)
+
     def _classify_event(
         self, topic: str, payload: dict[str, Any]
     ) -> tuple[str | None, str | None, str | None]: