- Add periodic note reindexing: Detect and reindex notes manually added to
  the notes directory (runs hourly, on startup, and after wake)
- Add orphaned screenshot registration: Register screenshot files from disk
  that aren't in the database before summarizing
- Add "Run Backfill" button in menu bar for manual backfill trigger
- Compute embeddings automatically for newly indexed notes

This fixes issues where:
- Users manually add notes to the notes directory
- Screenshots are captured but database entry fails
- Database is reset but files remain on disk

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>