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/participant_registry.py

@@ -0,0 +1,89 @@
+"""Participant Registry — Andockpunkt für Plugins als Teilnehmer."""
+
+from __future__ import annotations
+
+import logging
+from abc import ABC, abstractmethod
+from typing import Any
+
+logger = logging.getLogger(__name__)
+
+
+class ParticipantHandler(ABC):
+    """Interface that plugins implement to act as a conversation participant."""
+
+    @abstractmethod
+    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:
+        """Called when a new message arrives in a conversation this participant is part of.
+
+        Args:
+            conversation_id: UUID of the conversation.
+            message: The new message dict.
+            conversation: Full conversation dict with participants.
+            mentions: Parsed @mention participant types.
+            context: Tenant, user, and other context.
+
+        Returns:
+            Optional list of new message dicts (e.g. AI response).
+            For passive readers (system) → return None.
+            For reactive participants (ai) → return [message_dict, ...].
+        """
+        pass
+
+    @abstractmethod
+    def get_participant_info(self) -> dict[str, Any]:
+        """Return metadata: display_name, avatar_url, capabilities, description."""
+        pass
+
+
+class ParticipantRegistry:
+    """Global registry for plugin participants."""
+
+    def __init__(self) -> None:
+        self._handlers: dict[str, ParticipantHandler] = {}
+
+    def register(self, participant_type: str, handler: ParticipantHandler) -> None:
+        """Register a plugin as a participant type."""
+        self._handlers[participant_type] = handler
+        logger.info(f"Participant registered: {participant_type}")
+
+    def unregister(self, participant_type: str) -> None:
+        """Unregister a participant type."""
+        handler = self._handlers.pop(participant_type, None)
+        if handler:
+            logger.info(f"Participant unregistered: {participant_type}")
+
+    def get_handler(self, participant_type: str) -> ParticipantHandler | None:
+        """Get the handler for a participant type."""
+        return self._handlers.get(participant_type)
+
+    def list_types(self) -> list[str]:
+        """List all registered participant types."""
+        return list(self._handlers.keys())
+
+    def get_all_info(self) -> dict[str, dict[str, Any]]:
+        """Get info for all registered participants."""
+        return {ptype: handler.get_participant_info() for ptype, handler in self._handlers.items()}
+
+
+# Global instance
+_registry = ParticipantRegistry()
+
+
+def get_participant_registry() -> ParticipantRegistry:
+    """Get the global participant registry."""
+    return _registry
+
+
+def reset_participant_registry_for_testing() -> ParticipantRegistry:
+    """Create a fresh registry for testing."""
+    global _registry
+    _registry = ParticipantRegistry()
+    return _registry