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/blob_chunks.go

@@ -121,3 +121,32 @@ func (r *BlobChunkRepository) GetByChunkHashTx(ctx context.Context, tx *sql.Tx,
 	LogSQL("GetByChunkHashTx", "Found blob", chunkHash, "blob", bc.BlobID)
 	return &bc, nil
 }
+
+// DeleteOrphaned deletes blob_chunks entries where either the blob or chunk no longer exists
+func (r *BlobChunkRepository) DeleteOrphaned(ctx context.Context) error {
+	// Delete blob_chunks where the blob doesn't exist
+	query1 := `
+		DELETE FROM blob_chunks 
+		WHERE NOT EXISTS (
+			SELECT 1 FROM blobs 
+			WHERE blobs.id = blob_chunks.blob_id
+		)
+	`
+	if _, err := r.db.ExecWithLog(ctx, query1); err != nil {
+		return fmt.Errorf("deleting blob_chunks with missing blobs: %w", err)
+	}
+
+	// Delete blob_chunks where the chunk doesn't exist
+	query2 := `
+		DELETE FROM blob_chunks 
+		WHERE NOT EXISTS (
+			SELECT 1 FROM chunks 
+			WHERE chunks.chunk_hash = blob_chunks.chunk_hash
+		)
+	`
+	if _, err := r.db.ExecWithLog(ctx, query2); err != nil {
+		return fmt.Errorf("deleting blob_chunks with missing chunks: %w", err)
+	}
+
+	return nil
+}