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

alembic/versions/0137_drop_ai_chat_tables.py

@@ -0,0 +1,34 @@
+"""Drop AI chat tables (migrated to comm conversations)
+
+Revision ID: 0137
+Revises: 0136
+Create Date: 2026-08-21
+
+AI chat functionality is now handled by the kommunikation plugin's
+comm_conversations and comm_messages tables. The old AI-specific tables
+(ai_chat_sessions, ai_chat_messages, ai_chat_attachments, ai_conversations,
+ai_messages) are no longer needed and are dropped.
+"""
+
+from alembic import op
+import sqlalchemy as sa
+
+# revision identifiers, used by Alembic.
+revision = "0137"
+down_revision = "0136"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    # Use IF EXISTS to avoid errors if tables are already gone
+    op.execute("DROP TABLE IF EXISTS ai_chat_attachments CASCADE")
+    op.execute("DROP TABLE IF EXISTS ai_chat_messages CASCADE")
+    op.execute("DROP TABLE IF EXISTS ai_chat_sessions CASCADE")
+    op.execute("DROP TABLE IF EXISTS ai_messages CASCADE")
+    op.execute("DROP TABLE IF EXISTS ai_conversations CASCADE")
+
+
+def downgrade() -> None:
+    # Tables cannot be restored — data was migrated or was empty.
+    pass