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/base.py

@@ -30,6 +30,14 @@ class BasePlugin(ABC):
             raise ValueError(f"{self.__class__.__name__} must define a 'manifest' attribute")
         self._event_handlers: dict[str, Any] = {}
         self._routers: list[APIRouter] = []
+        self._container: Any = None  # ServiceContainer, set during on_activate
+
+    @property
+    def services(self) -> Any:
+        """Access the ServiceContainer after activation."""
+        if self._container is None:
+            raise RuntimeError("Services not available — plugin not activated")
+        return self._container
 
     # ─── Lifecycle Hooks ───
 
@@ -52,6 +60,7 @@ class BasePlugin(ABC):
             handler = self._make_event_handler(event_name)
             self._event_handlers[event_name] = handler
             event_bus.subscribe(event_name, handler)
+        self._container = service_container
 
     async def on_deactivate(
         self, db: AsyncSession, service_container: ServiceContainer, event_bus: EventBus