feat(python): share security state through Redis

[cport1]

Summary

• implement the shared Redis security-state contract for Python
• cover PoW, token replay, Siteverify idempotency, rate limits, suspicion, fingerprint cardinality, and site-key rotation
• use the same keys/TTLs and fail-closed behavior as Go and Node
• add a two-replica black-box Redis conformance workflow for all three production images
• update README, installation, Helm, and conformance documentation

Stack

Depends on #46, which depends on #45.

Validation

• Python: 91/91 unit tests
• live Python server: 12/12 API conformance checks
• python3 -m py_compile server.py redis_state.py
• multi-replica Docker/Redis matrix runs in CI (local Docker daemon unavailable)