feat: Unified Messaging System — kommunikation plugin, AI/Proactive/System participants, MessageSidebar, Rich Content Renderer

Phase 1: Backend plugin kommunikation (13 files, 10 tables, REST API, WebSocket, RBAC, DMS Bridge, Participant Registry, Mini-App Registry, Search Provider)
Phase 2: AI plugins as participants (ai_assistant + ai_proactive dock as participants, heartbeat job)
Phase 3: system_notif plugin (system events → chat messages, pinned System room)
Phase 4: Frontend MessageSidebar (replaces AISidebar, same design, comm API client, WebSocket hook, commStore)
Phase 5: Rich Content Block Renderer (11 components: Markdown, HTML, Image, Audio, Video, File, ActionCard, ContactCard, MiniApp, BlockRenderer)
BasePlugin: added services property + _container in on_activate

app/plugins/builtins/ai_assistant/plugin.py

@@ -42,12 +42,78 @@ class AIAssistantPlugin(BasePlugin):
         is_core=True,
     )
 
+    def __init__(self) -> None:
+        super().__init__()
+        self._ai_handler = None
+        self._msg_handler = None
+
     async def on_install(self, db, service_container) -> None:
         """Seed default provider and agent."""
         from app.plugins.builtins.ai_assistant.services import seed_defaults
 
         await seed_defaults(db)
 
+    async def on_activate(self, db, service_container, event_bus) -> None:
+        """Activate plugin: register context tools and participant handler."""
+        await super().on_activate(db, service_container, event_bus)
+
+        # Register as participant in the kommunikation system
+        try:
+            from app.plugins.builtins.ai_assistant.participant_handler import (
+                AIParticipantHandler,
+            )
+            from app.plugins.builtins.kommunikation.participant_registry import (
+                get_participant_registry,
+            )
+
+            self._ai_handler = AIParticipantHandler(service_container)
+            get_participant_registry().register("ai", self._ai_handler)
+            logger.info("AI Assistant registered as participant 'ai'")
+        except Exception:
+            logger.exception("Failed to register AI Assistant as participant")
+
+        # Subscribe to message.received events
+        try:
+            self._msg_handler = self._on_message_received
+            event_bus.subscribe("message.received", self._msg_handler)
+            logger.info("AI Assistant subscribed to message.received events")
+        except Exception:
+            logger.exception("Failed to subscribe to message.received events")
+
+    async def _on_message_received(self, payload: dict[str, Any]) -> None:
+        """Handle message.received event by delegating to the AI participant handler."""
+        if self._ai_handler is None:
+            return
+        try:
+            await self._ai_handler.handle_event(payload)
+        except Exception:
+            logger.exception("Error in AI participant handler for message.received")
+
+    async def on_deactivate(self, db, service_container, event_bus) -> None:
+        """Deactivate plugin: unregister participant and event subscriptions."""
+        # Unregister from participant registry
+        try:
+            from app.plugins.builtins.kommunikation.participant_registry import (
+                get_participant_registry,
+            )
+
+            get_participant_registry().unregister("ai")
+            logger.info("AI Assistant unregistered as participant 'ai'")
+        except Exception:
+            logger.exception("Failed to unregister AI Assistant as participant")
+
+        # Unsubscribe from message.received events
+        if self._msg_handler:
+            try:
+                event_bus.unsubscribe("message.received", self._msg_handler)
+            except Exception:
+                logger.exception("Failed to unsubscribe from message.received events")
+            self._msg_handler = None
+
+        self._ai_handler = None
+
+        await super().on_deactivate(db, service_container, event_bus)
+
     def get_notification_types(self) -> list[dict[str, Any]]:
         return [
             {