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/cascade_debug_test.go

@@ -43,7 +43,6 @@ func TestCascadeDeleteDebug(t *testing.T) {
 	for i := 0; i < 3; i++ {
 		chunk := &Chunk{
 			ChunkHash: fmt.Sprintf("cascade-chunk-%d", i),
-			SHA256:    fmt.Sprintf("cascade-sha-%d", i),
 			Size:      1024,
 		}
 		err = repos.Chunks.Create(ctx, nil, chunk)