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")

app/plugins/builtins/contracts.py

@@ -59,20 +59,31 @@ class ContractRegistry:
             cls._instance = super().__new__(cls)
             cls._instance._contracts: dict[str, Any] = {}
             cls._instance._loaded: set[str] = set()
+            cls._instance._unregistered: set[str] = set()
         return cls._instance
 
     # ─── registration ───
 
     def register(self, plugin_name: str, contract: Any) -> None:
-        """Register or replace a contract for a plugin."""
+        """Register or replace a contract for a plugin.
+
+        Clears the unregistered marker so a later deactivation can be
+        distinguished from a fresh lazy-load again (ARCH-014).
+        """
+        self._unregistered.discard(plugin_name)
         self._contracts[plugin_name] = contract
         self._loaded.add(plugin_name)
         logger.debug("Contract registered for plugin '%s'", plugin_name)
 
     def unregister(self, plugin_name: str) -> None:
-        """Remove a contract (e.g. when the plugin is deactivated)."""
+        """Remove a contract (e.g. when the plugin is deactivated).
+
+        Marks the plugin as explicitly unregistered so later ``get_contract``
+        calls cannot resurrect the contract via lazy-loading (ARCH-014).
+        """
         self._contracts.pop(plugin_name, None)
         self._loaded.discard(plugin_name)
+        self._unregistered.add(plugin_name)
 
     # ─── lookup ───
 
@@ -85,6 +96,11 @@ class ContractRegistry:
         if plugin_name in self._contracts:
             return self._contracts[plugin_name]
 
+        # Explicitly unregistered (deactivated): never resurrect via
+        # lazy-loading (ARCH-014) — the deactivated contract must stay gone.
+        if plugin_name in self._unregistered:
+            return None
+
         if plugin_name not in self._loaded:
             self._try_lazy_load(plugin_name)