fix(ci): resolve all mypy errors blocking CI

- mcp/__init__.py: remove dead MCP_TOOLS import and MaestroMCPServer back-compat alias; export MUSEHUB_TOOLS / MUSEHUB_TOOL_NAMES instead - protocol/hash.py, protocol/endpoints.py: rename MCP_TOOLS → MUSEHUB_TOOLS (mcp/tools package never exported MCP_TOOLS; it was renamed during the MuseHub extraction and callers were left behind) - ui_notifications.py: widen _fmt_relative signature to str | datetime (Postgres returns datetime objects; SQLite returns strings — the isinstance branch was correct but the annotation was too narrow) - auth/tokens.py: cast jwt.encode() return to str and payload.get("sub") to str | None (PyJWT lacks stubs; mypy inferred Any) - main.py: cast _rate_limit_exceeded_handler return to Response (slowapi lacks stubs; mypy inferred Any)