…ementation

Remove deadpool-libsql and libsql dependencies from mq-web-api.
Replace the database-backed rate limiter with a pure in-memory
HashMap protected by tokio::sync::Mutex, simplifying the
implementation and eliminating external database requirements.

- Remove database_url, database_auth_token, pool_max_size, and
  pool_timeout_seconds fields from RateLimitConfig
- Replace Pool with Mutex<HashMap<String, RateLimitEntry>> in RateLimiter
- Make RateLimiter::new() synchronous (no I/O setup needed)
- Remove DATABASE_URL, DATABASE_TOKEN, RATE_LIMIT_POOL_MAX_SIZE,
  and RATE_LIMIT_POOL_TIMEOUT_SECONDS env var handling from config
- Update tests to use in-memory helper instead of raw SQL inserts