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/miniapp_registry.py

@@ -0,0 +1,73 @@
+"""Mini-App registry for plugin-provided interactive chat components."""
+
+from __future__ import annotations
+
+import logging
+from typing import Any, Callable, Awaitable
+
+from pydantic import BaseModel, Field
+
+logger = logging.getLogger(__name__)
+
+
+class MiniAppDef(BaseModel):
+    """Definition of a mini-app that plugins can register."""
+
+    app_id: str = Field(..., description="Unique app identifier")
+    name: str = Field(..., description="Display name")
+    icon: str = Field(default="app", description="Icon name")
+    description: str = Field(default="", description="App description")
+    plugin_name: str = Field(..., description="Plugin that registered this app")
+    render_schema: dict[str, Any] = Field(
+        default_factory=dict, description="JSON schema for frontend rendering"
+    )
+
+
+class MiniAppRegistry:
+    """Registry for mini-apps that plugins provide for chat embedding."""
+
+    def __init__(self) -> None:
+        self._apps: dict[str, MiniAppDef] = {}
+
+    def register(
+        self,
+        app_id: str,
+        name: str,
+        icon: str,
+        description: str,
+        plugin_name: str,
+        render_schema: dict[str, Any] | None = None,
+    ) -> None:
+        """Register a mini-app."""
+        app = MiniAppDef(
+            app_id=app_id,
+            name=name,
+            icon=icon,
+            description=description,
+            plugin_name=plugin_name,
+            render_schema=render_schema or {},
+        )
+        self._apps[app_id] = app
+        logger.info(f"Mini-app registered: {app_id} by {plugin_name}")
+
+    def unregister(self, app_id: str) -> None:
+        """Unregister a mini-app."""
+        app = self._apps.pop(app_id, None)
+        if app:
+            logger.info(f"Mini-app unregistered: {app_id}")
+
+    def unregister_plugin(self, plugin_name: str) -> None:
+        """Unregister all mini-apps from a specific plugin."""
+        to_remove = [app_id for app_id, app in self._apps.items() if app.plugin_name == plugin_name]
+        for app_id in to_remove:
+            self._apps.pop(app_id, None)
+        if to_remove:
+            logger.info(f"Unregistered {len(to_remove)} mini-apps from plugin {plugin_name}")
+
+    def list_apps(self) -> list[dict[str, Any]]:
+        """List all available mini-apps for frontend."""
+        return [app.model_dump() for app in self._apps.values()]
+
+    def get_app(self, app_id: str) -> MiniAppDef | None:
+        """Get a specific mini-app definition."""
+        return self._apps.get(app_id)