feat: API key routes, search endpoint, conversation type wiring

- GET/POST/DELETE /api/api-keys blueprint registered
- GET /api/search?q=&content_type=&limit= semantic search endpoint
- create_conversation gains conversation_type param (default "chat")
- cleanup_old_conversations excludes mcp type from retention sweep
- POST /api/chat/conversations accepts conversation_type body field

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/fabledassistant/services/chat.py

@@ -15,10 +15,10 @@ logger = logging.getLogger(__name__)
 
 
 async def create_conversation(
-    user_id: int, title: str = "", model: str = ""
+    user_id: int, title: str = "", model: str = "", conversation_type: str = "chat"
 ) -> Conversation:
     async with async_session() as session:
-        conv = Conversation(user_id=user_id, title=title, model=model)
+        conv = Conversation(user_id=user_id, title=title, model=model, conversation_type=conversation_type)
         session.add(conv)
         await session.commit()
         # Re-fetch with messages eagerly loaded to avoid lazy-load in async context
@@ -128,7 +128,11 @@ async def cleanup_old_conversations(user_id: int, days: int) -> int:
     async with async_session() as session:
         result = await session.execute(
             sa_delete(Conversation)
-            .where(Conversation.user_id == user_id, Conversation.updated_at < cutoff)
+            .where(
+                Conversation.user_id == user_id,
+                Conversation.updated_at < cutoff,
+                Conversation.conversation_type != "mcp",  # preserve MCP audit trail
+            )
             .returning(Conversation.id)
         )
         await session.commit()