feat: library seed and daily prune scan

[retardgerman]

Summary

• On first boot, scans the entire Jellyfin library via the API and pre-populates the existing dedup store, so pre-existing content never triggers a "new item" Discord notification.
• Adds a daily background prune scan that removes dedup keys for items deleted from Jellyfin, preventing unbounded growth.
• Adds a "Re-Seed Library" button in the dashboard to manually re-run the seed scan.
• Both scans abort/retry on incomplete Jellyfin fetches rather than acting on partial data.
• CHANGELOG updated, version bumped to 1.5.6.

Docs and code were AI-assisted; all changes manually reviewed and verified.