Tune Django settings for highload readiness

[ak4code]

• Enable native psycopg connection pooling for PostgreSQL (DJANGO_DB_POOL, min/max size and timeout via env); fall back to persistent connections with health checks for other engines
• Move sessions to cached_db backend: reads served from Redis, write-through to DB so sessions survive cache flush
• Tune Redis cache client: connection pool size, socket timeouts, retry_on_timeout, health checks, configurable key prefix
• Switch default password hasher to Argon2 (argon2-cffi), keeping legacy hashers for transparent migration of existing passwords
• Add UserRateThrottle alongside AnonRateThrottle with env-driven rates
• Set NUM_PROXIES in prod for correct client IP behind reverse proxy
• Reduce 4xx log noise via django.request logger level (default ERROR)
• Tune gunicorn: workers default to CPU*2+1, gthread worker class with configurable threads, /dev/shm heartbeat dir, backlog 2048, graceful timeout, keep-alive 5
• Document new env vars in .env.example and README