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/kommunikation/websocket_manager.py

@@ -0,0 +1,89 @@
+"""WebSocket connection manager for the kommunikation plugin."""
+
+from __future__ import annotations
+
+import json
+import logging
+from typing import Any
+
+from fastapi import WebSocket
+
+logger = logging.getLogger(__name__)
+
+
+class WebSocketManager:
+    """Manages WebSocket connections per user for real-time messaging."""
+
+    def __init__(self) -> None:
+        # user_id (str) → list of WebSocket connections
+        self._connections: dict[str, list[WebSocket]] = {}
+        # conversation_id (str) → set of user_ids subscribed
+        self._subscriptions: dict[str, set[str]] = {}
+
+    async def connect(self, websocket: WebSocket, user_id: str) -> None:
+        """Accept and register a new WebSocket connection."""
+        await websocket.accept()
+        if user_id not in self._connections:
+            self._connections[user_id] = []
+        self._connections[user_id].append(websocket)
+        logger.debug(f"WebSocket connected: user={user_id}, total={len(self._connections[user_id])}")
+
+    async def disconnect(self, websocket: WebSocket, user_id: str) -> None:
+        """Remove a WebSocket connection."""
+        conns = self._connections.get(user_id, [])
+        if websocket in conns:
+            conns.remove(websocket)
+        if not conns:
+            self._connections.pop(user_id, None)
+            # Remove from all subscriptions
+            for conv_id, users in self._subscriptions.items():
+                users.discard(user_id)
+        logger.debug(f"WebSocket disconnected: user={user_id}, remaining={len(conns)}")
+
+    def subscribe(self, conversation_id: str, user_id: str) -> None:
+        """Subscribe a user to a conversation's updates."""
+        if conversation_id not in self._subscriptions:
+            self._subscriptions[conversation_id] = set()
+        self._subscriptions[conversation_id].add(user_id)
+
+    def unsubscribe(self, conversation_id: str, user_id: str) -> None:
+        """Unsubscribe a user from a conversation."""
+        if conversation_id in self._subscriptions:
+            self._subscriptions[conversation_id].discard(user_id)
+
+    async def send_to_user(self, user_id: str, message: dict[str, Any]) -> None:
+        """Send a message to all connections of a specific user."""
+        conns = self._connections.get(user_id, [])
+        text = json.dumps(message, default=str)
+        for ws in conns:
+            try:
+                await ws.send_text(text)
+            except Exception:
+                logger.warning(f"Failed to send to user {user_id}, removing connection")
+                await self.disconnect(ws, user_id)
+
+    async def send_to_conversation(
+        self,
+        conversation_id: str,
+        message: dict[str, Any],
+        exclude_user: str | None = None,
+    ) -> None:
+        """Send a message to all users subscribed to a conversation."""
+        user_ids = self._subscriptions.get(conversation_id, set())
+        for user_id in list(user_ids):
+            if exclude_user and user_id == exclude_user:
+                continue
+            await self.send_to_user(user_id, message)
+
+    async def broadcast(self, message: dict[str, Any]) -> None:
+        """Broadcast a message to all connected users."""
+        for user_id in list(self._connections.keys()):
+            await self.send_to_user(user_id, message)
+
+    def get_online_users(self) -> list[str]:
+        """Get list of currently connected user IDs."""
+        return list(self._connections.keys())
+
+    def is_user_online(self, user_id: str) -> bool:
+        """Check if a user has any active connections."""
+        return user_id in self._connections and len(self._connections[user_id]) > 0