fix(cache): reconnect Redis after startup timeout

[germondai]

Summary

• replace the permanent one-shot Redis failure state with background session-cache recovery
• bound each attempt with REDIS_CONNECT_TIMEOUT_MS, defaulting to 5 seconds
• retry after REDIS_RETRY_DELAY_MS, defaulting to 5 seconds, with zero disabling retries
• activate Tier 2 automatically once a later connection succeeds
• cancel pending retries and close the Redis client during shutdown
• keep the cacheless minimal Compose deployment free of a retry loop
• update configuration, Compose files, troubleshooting, architecture docs, and changelog

Verification

• bun run verify
• 363 tests passed
• lint, typecheck, web build, and docs build passed

Fixes #92