fix(arch-014,arch-020): no contract lazy-resurrect after unregister; event bus dedupes handlers

Also fixes ARCH-029/041: none-check before attribute access in trigger dispatcher.

app/core/event_bus.py

@@ -36,8 +36,13 @@ class EventBus:
         self._handlers: dict[str, list[EventHandler]] = defaultdict(list)
 
     def subscribe(self, event_name: str, handler: EventHandler) -> None:
-        """Subscribe a handler to an event."""
-        self._handlers[event_name].append(handler)
+        """Subscribe a handler to an event.
+
+        Idempotent: subscribing the same handler twice is a no-op
+        (ARCH-020) so double activation cannot fire handlers twice.
+        """
+        if handler not in self._handlers[event_name]:
+            self._handlers[event_name].append(handler)
 
     def unsubscribe(self, event_name: str, handler: EventHandler) -> None:
         """Unsubscribe a handler from an event."""

app/core/trigger_dispatcher.py

@@ -121,11 +121,14 @@ class TriggerDispatcher:
         """Query DB for active automations matching *event_name* and dispatch."""
         from app.core.db import get_session_factory
         from app.plugins.builtins.contracts import get_contract
+        # None-check FIRST — accessing attributes on the contract before the
+        # check crashed with AttributeError when automation was inactive
+        # (ARCH-029/041).
         automation_contract = get_contract("automation")
-        AutomationDefinition = automation_contract.Automation  # noqa: N806
         if automation_contract is None:
             logger.debug("Automation plugin not available — trigger skipped")
             return
+        AutomationDefinition = automation_contract.Automation  # noqa: N806
 
         factory = get_session_factory()
         tenant_id = payload.get("tenant_id")