fix: bound cache follow lifetime and honor preferred backups

[zijiren233]

Summary

Cache-follow mappings could retain channel affinity for the full upstream retention period, and preferred backup-only channels were excluded until ordinary channels were exhausted.

• Cap newly written mappings at five minutes across prompt, user, and generic scopes, including upstream retention and configured TTLs. Preserve existing records without migration or additional read-time limits.
• Set the default recent mapping update debounce to 45 seconds. Backup-only channels write recent mappings only; ordinary channels continue writing stable and recent mappings.
• Allow eligible preferred backup-only channels in preference order before general backup activation. Health, availability, permission, and current retry-round failure filters still apply. Selecting a preferred backup does not unlock other backups.
• Update configuration descriptions and regression coverage for TTL bounds, recent-only recording, debounce, selection order, and retry budgets.

Validation

• Full core test suite: 2,079 tests passed across 100 packages.
• Translation JSON parsing and git diff whitespace checks passed.