feat: add MCP API endpoint and enhance API documentation

Add new MCP configuration management endpoint and enhance API documentation
with detailed descriptions, examples, and OpenAPI support for better
developer experience.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

backend/src/gateway/routers/artifacts.py

@@ -59,19 +59,40 @@ def is_text_file_by_content(path: Path, sample_size: int = 8192) -> bool:
         return False
 
 
-@router.get("/threads/{thread_id}/artifacts/{path:path}")
+@router.get(
+    "/threads/{thread_id}/artifacts/{path:path}",
+    summary="Get Artifact File",
+    description="Retrieve an artifact file generated by the AI agent. Supports text, HTML, and binary files.",
+)
 async def get_artifact(thread_id: str, path: str, request: Request) -> FileResponse:
     """Get an artifact file by its path.
 
+    The endpoint automatically detects file types and returns appropriate content types.
+    Use the `?download=true` query parameter to force file download.
+
     Args:
         thread_id: The thread ID.
         path: The artifact path with virtual prefix (e.g., mnt/user-data/outputs/file.txt).
+        request: FastAPI request object (automatically injected).
 
     Returns:
-        The file content as a FileResponse.
+        The file content as a FileResponse with appropriate content type:
+        - HTML files: Rendered as HTML
+        - Text files: Plain text with proper MIME type
+        - Binary files: Inline display with download option
 
     Raises:
-        HTTPException: 404 if file not found, 403 if access denied.
+        HTTPException:
+            - 400 if path is invalid or not a file
+            - 403 if access denied (path traversal detected)
+            - 404 if file not found
+
+    Query Parameters:
+        download (bool): If true, returns file as attachment for download
+
+    Example:
+        - Get HTML file: `/api/threads/abc123/artifacts/mnt/user-data/outputs/index.html`
+        - Download file: `/api/threads/abc123/artifacts/mnt/user-data/outputs/data.csv?download=true`
     """
     actual_path = _resolve_artifact_path(thread_id, path)