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/routes/chat.py

@@ -75,7 +75,11 @@ async def create_conversation_route():
     data = await request.get_json(force=True, silent=True) or {}
     title = data.get("title", "")
     model = data.get("model", Config.OLLAMA_MODEL)
-    conv = await create_conversation(uid, title=title, model=model)
+    conversation_type = data.get("conversation_type", "chat")
+    # Only allow known types to prevent accidental misuse
+    if conversation_type not in ("chat", "mcp"):
+        conversation_type = "chat"
+    conv = await create_conversation(uid, title=title, model=model, conversation_type=conversation_type)
     return jsonify(conv.to_dict()), 201