Epic: Audit P1: FastAPI backend quality

[VforVitorio]

FastAPI web backend audit (the layer the React SPA consumes). Full plan: docs/audits/AUDIT_P1_BACKEND.md. Part of #25. Phases 1-2 are hard dependencies of the migration; phase 1 overlaps packaging issues #27-#29.

Phases:

• P1: paths and packaging (walker fix(data): repo-root .git walker breaks bare-metal data paths #27, whisper fix(deps): openai-whisper build fails on a fresh venv #28, fastmcp fix(deps): reconcile fastmcp pyproject.toml vs requirements.txt drift #29, Dockerfile reload + FastF1 cache)
• P2: contract freeze (kill dead endpoints, operation_ids, response models, error envelope, publish 38-route spec) before TS type generation
• P3: event-loop correctness (LLM/STT to-thread + timeout, 8 FastF1 routes async to def)
• P4: behavior/validation fixes (TTS rate, upload caps, simulate pre-validation, SSE framing)
• P5: dead-code sweep
• P6: typing and contract tests
• P7: perf and structure