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/chunks_test.go

@@ -15,7 +15,6 @@ func TestChunkRepository(t *testing.T) {
 	// Test Create
 	chunk := &Chunk{
 		ChunkHash: "chunkhash123",
-		SHA256:    "sha256hash123",
 		Size:      4096,
 	}
 
@@ -35,9 +34,6 @@ func TestChunkRepository(t *testing.T) {
 	if retrieved.ChunkHash != chunk.ChunkHash {
 		t.Errorf("chunk hash mismatch: got %s, want %s", retrieved.ChunkHash, chunk.ChunkHash)
 	}
-	if retrieved.SHA256 != chunk.SHA256 {
-		t.Errorf("sha256 mismatch: got %s, want %s", retrieved.SHA256, chunk.SHA256)
-	}
 	if retrieved.Size != chunk.Size {
 		t.Errorf("size mismatch: got %d, want %d", retrieved.Size, chunk.Size)
 	}
@@ -51,7 +47,6 @@ func TestChunkRepository(t *testing.T) {
 	// Test GetByHashes
 	chunk2 := &Chunk{
 		ChunkHash: "chunkhash456",
-		SHA256:    "sha256hash456",
 		Size:      8192,
 	}
 	err = repo.Create(ctx, nil, chunk2)