fix(docker): authenticate rate limit Redis storage

[harshmathurx]

Summary

• add shared Redis URL helpers for the Docker server
• include explicit Redis ACL username + URL-encoded password in the app Redis URL
• reuse the configured Redis credentials for unauthenticated Redis rate-limit storage URIs
• add regression coverage for protected Redis + SlowAPI storage URI handling

Why

The Docker entrypoint protects the in-container Redis with REDIS_PASSWORD, but older/self-managed configs can still point rate_limiting.storage_uri at redis://localhost:6379 without credentials. SlowAPI/limits then touches Redis before the route handler and fails during the Redis HELLO handshake.

Fixes #2040.

Test plan

• python3 -m py_compile deploy/docker/redis_config.py deploy/docker/server.py deploy/docker/tests/test_security_default_posture.py
• PYTHONPATH=deploy/docker uv run --no-project --with pytest pytest deploy/docker/tests/test_security_default_posture.py -k 'redis_url_includes_acl_username or rate_limit_redis_storage_reuses_redis_password or rate_limit_storage_preserves_explicit_auth or rate_limit_storage_keeps_memory_backend' -q
• PYTHONPATH=deploy/docker uv run --no-project --with pytest pytest deploy/docker/tests/test_security_container_posture.py -k 'not sandbox' -q

Note: full Docker/runtime reproduction was not possible in this environment because the local Docker daemon is not running.