Fix foreign key constraints and improve snapshot tracking

- Add unified compression/encryption package in internal/blobgen
- Update DATAMODEL.md to reflect current schema implementation
- Refactor snapshot cleanup into well-named methods for clarity
- Add snapshot_id to uploads table to track new blobs per snapshot
- Fix blob count reporting for incremental backups
- Add DeleteOrphaned method to BlobChunkRepository
- Fix cleanup order to respect foreign key constraints
- Update tests to reflect schema changes

internal/database/files.go

@@ -28,7 +28,6 @@ func (r *FileRepository) Create(ctx context.Context, tx *sql.Tx, file *File) err
 		INSERT INTO files (id, path, mtime, ctime, size, mode, uid, gid, link_target)
 		VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?)
 		ON CONFLICT(path) DO UPDATE SET
-			id = excluded.id,
 			mtime = excluded.mtime,
 			ctime = excluded.ctime,
 			size = excluded.size,