Stash Metadata v0.3.68

Adds a read-only completed-play backfill preview without JAVBeacon, available as a manual task and admin API route. Exact file identity, session deduplication, bounded coverage and incomplete-history reporting prevent misleading backfill candidates. No activity is written. Includes Linux amd64/arm64 and macOS arm64 binaries plus the unchanged companion and userscript. Validation: Go tests and vet, companion Python tests, browser JavaScript tests. See CHANGELOG.md for intervening fixes since v0.3.63.