Add rate limiting and lifespan HTTP clients

[MyDrift-user]

Type of Change

• Refactoring
• Security patch

Description

• Introduced login rate limiting with slowapi and central Limiter (src/mw_utils/limiter.py), expose LOGIN_RATE_LIMIT env var, and attach middleware/exception handler in app.
• Refactored files and office_preview routes to use lifespan-managed httpx clients (init/close in app.lifespan) to improve connection pooling and resource cleanup.
• Removed Postgres service and DB env vars from compose/.env.example and stop using a local DB connection; keep Redis config.
• Add slowapi to requirements and update secure_auth.login to accept Request and apply the limiter decorator. Misc: remove unused imports and session cleanup task; ensure HTTP clients raise if not initialized.