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_comprehensive_test.go

@@ -195,12 +195,10 @@ func TestOrphanedChunkCleanup(t *testing.T) {
 	// Create chunks
 	chunk1 := &Chunk{
 		ChunkHash: "orphaned-chunk",
-		SHA256:    "orphaned-chunk-sha",
 		Size:      1024,
 	}
 	chunk2 := &Chunk{
 		ChunkHash: "referenced-chunk",
-		SHA256:    "referenced-chunk-sha",
 		Size:      1024,
 	}
 
@@ -363,7 +361,6 @@ func TestFileChunkRepositoryWithUUIDs(t *testing.T) {
 	for i, chunkHash := range chunks {
 		chunk := &Chunk{
 			ChunkHash: chunkHash,
-			SHA256:    fmt.Sprintf("sha-%s", chunkHash),
 			Size:      1024,
 		}
 		err = repos.Chunks.Create(ctx, nil, chunk)
@@ -447,7 +444,6 @@ func TestChunkFileRepositoryWithUUIDs(t *testing.T) {
 	// Create a chunk that appears in both files (deduplication)
 	chunk := &Chunk{
 		ChunkHash: "shared-chunk",
-		SHA256:    "shared-chunk-sha",
 		Size:      1024,
 	}
 	err = repos.Chunks.Create(ctx, nil, chunk)
@@ -694,7 +690,6 @@ func TestCascadeDelete(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)