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/uploads.go

@@ -11,6 +11,7 @@ import (
 // Upload represents a blob upload record
 type Upload struct {
 	BlobHash   string
+	SnapshotID string
 	UploadedAt time.Time
 	Size       int64
 	DurationMs int64
@@ -29,15 +30,15 @@ func NewUploadRepository(conn *sql.DB) *UploadRepository {
 // Create inserts a new upload record
 func (r *UploadRepository) Create(ctx context.Context, tx *sql.Tx, upload *Upload) error {
 	query := `
-		INSERT INTO uploads (blob_hash, uploaded_at, size, duration_ms)
-		VALUES (?, ?, ?, ?)
+		INSERT INTO uploads (blob_hash, snapshot_id, uploaded_at, size, duration_ms)
+		VALUES (?, ?, ?, ?, ?)
 	`
 
 	var err error
 	if tx != nil {
-		_, err = tx.ExecContext(ctx, query, upload.BlobHash, upload.UploadedAt, upload.Size, upload.DurationMs)
+		_, err = tx.ExecContext(ctx, query, upload.BlobHash, upload.SnapshotID, upload.UploadedAt, upload.Size, upload.DurationMs)
 	} else {
-		_, err = r.conn.ExecContext(ctx, query, upload.BlobHash, upload.UploadedAt, upload.Size, upload.DurationMs)
+		_, err = r.conn.ExecContext(ctx, query, upload.BlobHash, upload.SnapshotID, upload.UploadedAt, upload.Size, upload.DurationMs)
 	}
 
 	return err
@@ -133,3 +134,14 @@ type UploadStats struct {
 	MinDurationMs int64
 	MaxDurationMs int64
 }
+
+// GetCountBySnapshot returns the count of uploads for a specific snapshot
+func (r *UploadRepository) GetCountBySnapshot(ctx context.Context, snapshotID string) (int64, error) {
+	query := `SELECT COUNT(*) FROM uploads WHERE snapshot_id = ?`
+	var count int64
+	err := r.conn.QueryRowContext(ctx, query, snapshotID).Scan(&count)
+	if err != nil {
+		return 0, err
+	}
+	return count, nil
+}