Simplify retry decorator and add builder dependency group

Add retry logic for explainxkcd 503 errors (#2)

simplify: use FastMCP custom_route for landing page

fix: use Starlette directly to avoid FastAPI trailing slash redirect

feat: add browser landing page for MCP endpoint

docs: add sync comments for EMBED_MODEL across builder, Dockerfile, workflow

refactor: simplify codebase

fix: lint issues and make tests compatible with deployed index

fix: add module docstring to __init__.py for uv_build compatibility

refactor: simplify cleanup from /simplify review

migrate to Hugging Face Spaces; consolidate builder module

fix(server): don't let transient refresh errors kill the poll thread

refactor: avoid duplicate int() cast in get_comic

refactor: drop unused upload_release; workflow uses gh CLI directly

feat: add get_comic tool for direct lookups by number

feat: resilient indexer with retry, limit, and incremental cache

refactor: apply /simplify review

feat: add xkcd semantic search MCP server with daily indexing