Douyin_TikTok_Download_API v5 rewrite: self-healing identity pool, full archiving, and MCP integration

v5 is rewritten from an empty branch and inherits no v4 code. Identity sourcing shifts from manually filled browser cookies in config.yaml to headless-browser auto-generated guest identities, replenished when availability runs low; the request egress adds health grading, per-identity in-flight locks, per-identity-and-endpoint token buckets, and per-endpoint circuit breakers; async returns 202 with a task_id by default, or ?wait= for synchronous. Parsed results are written to PostgreSQL and Redis and persist even after upstream deletes; auth becomes scoped, role-based API keys. Beyond REST, entry points add MCP (streamable-http) and CLI, and the console moves to React. Deployment is just docker compose up.