Switch server/app.py to create_fastapi_app and simplify /tasks response

Fix Phase 2 'Not enough tasks with graders' — add canonical TASKS registry

Fix Phase 2: add GET /tasks and POST /grader endpoints

Re-enable Gradio web UI at / for friendly browser-facing Space page

Replace MCPEnvironment with canonical Environment base class

Add MCPEnvironment subclass with 5 dispatcher tools and FastAPI server