Add Valkey caching for EnginX journal lookup

[Dagonite]

Closes #509.

Description

• Add a small Valkey cache helper for JSON get/set operations.
• Cache the EnginX journal run-to-cycle map in Valkey, while keeping the existing in-process lru_cache.
• Fall back to reading journal XML files if Valkey is unavailable or misses.
• Avoid caching an empty map when the journal directory is unavailable.
• Add tests for the cache helper and EnginX cache hit/miss behavior.

Testing

Install/update local deps

.\.venv\Scripts\python.exe -m pip install -e ".[test]"`

Run Valkey locally

docker run --rm -p 6379:6379 valkey/valkey:latest
$env:VALKEY_URL = "redis://localhost:6379/0"
$env:ENGINX_MAP_CACHE_TTL_SECONDS = "86400"