docs: caching strategy, key namespaces, and invalidation reference

[code-0-stella]

Description

Adds a cache strategy reference for the Redis-backed cache-aside layer and a drift test to keep the documentation aligned with the implementation.

What changed

• Expanded docs/cache.md with:
  • cache strategy overview
  • key namespace convention (org:{orgId}:{key})
  • TTL policy and cache versioning via CACHE_VERSION
  • invalidation triggers for vault and analytics writes
  • Redis fallback and in-memory LRU behavior
  • a checklist for safely adding new cached reads
• Added src/tests/docs.cache.test.ts to assert that the documentation references the documented namespace format, cache version, fallback behavior, and invalidation helpers.

Testing

• Ran:

  npm test -- src/tests/docs.cache.test.ts

  Closes: Author a caching strategy and key-namespace reference in docs/cache.md #886

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