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/repositories_test.go

@@ -34,7 +34,6 @@ func TestRepositoriesTransaction(t *testing.T) {
 		// Create chunks
 		chunk1 := &Chunk{
 			ChunkHash: "tx_chunk1",
-			SHA256:    "tx_sha1",
 			Size:      512,
 		}
 		if err := repos.Chunks.Create(ctx, tx, chunk1); err != nil {
@@ -43,7 +42,6 @@ func TestRepositoriesTransaction(t *testing.T) {
 
 		chunk2 := &Chunk{
 			ChunkHash: "tx_chunk2",
-			SHA256:    "tx_sha2",
 			Size:      512,
 		}
 		if err := repos.Chunks.Create(ctx, tx, chunk2); err != nil {
@@ -159,7 +157,6 @@ func TestRepositoriesTransactionRollback(t *testing.T) {
 		// Create a chunk
 		chunk := &Chunk{
 			ChunkHash: "rollback_chunk",
-			SHA256:    "rollback_sha",
 			Size:      1024,
 		}
 		if err := repos.Chunks.Create(ctx, tx, chunk); err != nil {