feat(UI-Overhaul-Phase2): AI Assistent in Kommunikation integriert

Migration 0137: Drop AI chat tables (ai_chat_sessions, ai_chat_messages, ai_chat_attachments, ai_conversations, ai_messages)

Backend:
- Remove AIChatSession, AIChatMessage, AIChatAttachment models from ai_assistant/models.py
- Remove AIConversation, AIMessage from app/models/__init__.py
- Remove session/message/stream/attachment routes from ai_assistant/routes.py
- Add new streaming route POST /ai/conversations/{conversation_id}/stream using comm tables
- Add new messages route GET /ai/conversations/{conversation_id}/messages using comm tables
- Add stream_chat_comm, get_comm_messages, save_comm_message to services.py
- Update external_api.py to use CommConversation/CommMessage instead of AIChatSession/AIChatMessage
- Update unified_search ai_chat_provider to search comm_messages with conversation_type=ai
- Remove ai_copilot router from main.py and routes/__init__.py
- Remove ai_conversation from entity_permissions.py and owner_transfer_service.py
- Update ai_assistant/plugin.py get_entity_models to remove AIChatSession
- Guard ai_copilot_service.py imports with try/except

Frontend:
- Remove AIAssistant.tsx, AIAssistantStandalone.tsx, SessionList.tsx, ChatWindow.tsx
- Remove AI Assistant routes from routes/index.tsx
- Update api/ai.ts: streamChat uses /ai/conversations/{id}/stream, fetchMessages uses /ai/conversations/{id}/messages
- Update Communication.tsx: use convId for AI streaming, remove aiSessionId, use fetchAiMessages for AI conversations
- Update AiChatPanel.tsx: create comm conversation instead of AI session, use new fetchMessages
- Update AISidebar.tsx: remove ChatWindow import, show placeholder

tsc clean, build successful, backend import OK

frontend/src/components/window/AiChatPanel.tsx

@@ -1,12 +1,8 @@
 import { asError } from '@/utils/errorTypes';
 import React, { useState, useRef, useEffect, useCallback } from 'react';
 import { Sparkles, Send } from 'lucide-react';
-import {
-  streamChat,
-  createSession,
-  fetchMessages,
-  type ChatMessage,
-} from '@/api/ai';
+import { streamChat, fetchMessages } from '@/api/ai';
+import { apiClient } from '@/api/client';
 
 interface AiChatPanelProps {
   windowTitle: string;
@@ -30,24 +26,30 @@ export function AiChatPanel({ windowTitle, windowType }: AiChatPanelProps) {
   const scrollRef = useRef<HTMLDivElement>(null);
   const msgIdCounter = useRef(0);
 
-  // Create a sidebar session on mount
+  // Create a comm conversation for AI chat on mount
   useEffect(() => {
     let cancelled = false;
     setLoadingSession(true);
-    createSession({ title: `KI: ${windowTitle}`, is_sidebar: true })
-      .then((session) => {
+    apiClient.post('/comm/conversations', {
+      title: `KI: ${windowTitle}`,
+      participant_ids: [],
+      is_direct: false,
+      metadata: { conversation_type: 'ai' },
+    })
+      .then((res) => {
         if (cancelled) return;
-        setSessionId(session.id);
-        return fetchMessages(session.id).then((msgs: ChatMessage[]) => {
+        const convId = res.data.id;
+        setSessionId(convId);
+        return fetchMessages(convId).then((msgs) => {
           if (cancelled) return;
           setMessages(
-            msgs.map((m) => ({ id: m.id, role: m.role, content: m.content }))
+            msgs.map((m, i) => ({ id: `msg-${i}`, role: m.role, content: m.content }))
           );
         });
       })
       .catch((e) => {
         if (!cancelled) {
-          console.error('AI session creation failed:', e);
+          console.error('AI conversation creation failed:', e);
           setError('KI Chat ist in diesem Kontext nicht verfügbar');
         }
       })
@@ -90,7 +92,7 @@ export function AiChatPanel({ windowTitle, windowType }: AiChatPanelProps) {
         } else if (event.type === 'done') {
           const msgs = await fetchMessages(sessionId);
           setMessages(
-            msgs.map((m) => ({ id: m.id, role: m.role, content: m.content }))
+            msgs.map((m, i) => ({ id: `msg-${i}`, role: m.role, content: m.content }))
           );
           setStreamingContent('');
         } else if (event.type === 'error') {