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/database_test.go

@@ -75,8 +75,8 @@ func TestDatabaseConcurrentAccess(t *testing.T) {
 
 	for i := 0; i < 10; i++ {
 		go func(i int) {
-			_, err := db.ExecWithLog(ctx, "INSERT INTO chunks (chunk_hash, sha256, size) VALUES (?, ?, ?)",
-				fmt.Sprintf("hash%d", i), fmt.Sprintf("sha%d", i), i*1024)
+			_, err := db.ExecWithLog(ctx, "INSERT INTO chunks (chunk_hash, size) VALUES (?, ?)",
+				fmt.Sprintf("hash%d", i), i*1024)
 			results <- result{index: i, err: err}
 		}(i)
 	}