Write apps/backend/docs/concepts-caching.md for lib/conversationCache.ts and the Redis cache keys in lib/redis.ts: what is cached, the key format, the TTL, and every path that invalidates it.
Acceptance criteria:
- Documents each cache key, its shape, TTL and payload
- Lists every call site of
invalidateConversationCaches and the event that triggers it
- Explains the degraded behaviour when Redis is unavailable — the cache is an optimisation, not a correctness dependency
- Documents the per-device scoping hazard: a conversation preview is device-specific, so a cache key that omitted the device would leak the wrong ciphertext to the wrong device
Write
apps/backend/docs/concepts-caching.mdforlib/conversationCache.tsand the Redis cache keys inlib/redis.ts: what is cached, the key format, the TTL, and every path that invalidates it.Acceptance criteria:
invalidateConversationCachesand the event that triggers it