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/repository_edge_cases_test.go

@@ -170,7 +170,6 @@ func TestDuplicateHandling(t *testing.T) {
 	t.Run("duplicate chunk hashes", func(t *testing.T) {
 		chunk := &Chunk{
 			ChunkHash: "duplicate-chunk",
-			SHA256:    "duplicate-sha",
 			Size:      1024,
 		}
 
@@ -204,7 +203,6 @@ func TestDuplicateHandling(t *testing.T) {
 
 		chunk := &Chunk{
 			ChunkHash: "test-chunk-dup",
-			SHA256:    "test-sha-dup",
 			Size:      1024,
 		}
 		err = repos.Chunks.Create(ctx, nil, chunk)