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/chunks_ext.go

@@ -7,7 +7,7 @@ import (
 
 func (r *ChunkRepository) List(ctx context.Context) ([]*Chunk, error) {
 	query := `
-		SELECT chunk_hash, sha256, size
+		SELECT chunk_hash, size
 		FROM chunks
 		ORDER BY chunk_hash
 	`
@@ -24,7 +24,6 @@ func (r *ChunkRepository) List(ctx context.Context) ([]*Chunk, error) {
 
 		err := rows.Scan(
 			&chunk.ChunkHash,
-			&chunk.SHA256,
 			&chunk.Size,
 		)
 		if err != nil {