Add background TTL-based cleanup (L2 safety net) directly into
FileMediaStore so file deletion and in-memory ref removal happen
atomically under the same mutex, preventing dangling references.
- Add storedAt timestamp and refToScope reverse map to mediaEntry
- Add CleanExpired() for atomic TTL-based expiration
- Add Start()/Stop() for background goroutine lifecycle
- Add MediaCleanupConfig (enabled, max_age, interval) to config
- Wire up in cmd_gateway.go with config-driven defaults
- Add 8 new tests including concurrent cleanup safety
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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