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/system_notif/participant_handler.py

@@ -0,0 +1,46 @@
+"""System participant handler — passive participant that only reads."""
+
+from __future__ import annotations
+
+import logging
+from typing import Any
+
+from app.plugins.builtins.kommunikation.participant_registry import ParticipantHandler
+
+logger = logging.getLogger(__name__)
+
+
+class SystemParticipantHandler(ParticipantHandler):
+    """System participant — passive, does not respond to messages.
+
+    The system participant only sends messages triggered by system events
+    (handled in the plugin's event handlers). It does not respond to
+    user messages in the chat.
+    """
+
+    def __init__(self, container: Any) -> None:
+        self._container = container
+
+    async def on_message_received(
+        self,
+        conversation_id: Any,
+        message: dict[str, Any],
+        conversation: dict[str, Any],
+        mentions: list[str],
+        context: dict[str, Any],
+    ) -> list[dict[str, Any]] | None:
+        """System is passive — does not respond to messages.
+
+        Users can interact with action_card buttons (open, archive) which
+        are handled via the frontend, not via new messages.
+        """
+        return None
+
+    def get_participant_info(self) -> dict[str, Any]:
+        """Return participant metadata."""
+        return {
+            "display_name": "System",
+            "avatar_url": None,
+            "capabilities": ["notifications", "action_cards"],
+            "description": "System-Benachrichtigungen",
+        }