Implement SQLite index database layer

- Add pure Go SQLite driver (modernc.org/sqlite) to avoid CGO dependency
- Implement database connection management with WAL mode
- Add write mutex for serializing concurrent writes
- Create schema for all tables matching DESIGN.md specifications
- Implement repository pattern for all database entities:
  - Files, FileChunks, Chunks, Blobs, BlobChunks, ChunkFiles, Snapshots
- Add transaction support with proper rollback handling
- Add fatal error handling for database integrity issues
- Add snapshot fields for tracking file sizes and compression ratios
- Make index path configurable via VAULTIK_INDEX_PATH environment variable
- Add comprehensive test coverage for all repositories
- Add format check to Makefile to ensure code formatting

internal/database/blob_chunks.go

@@ -24,7 +24,7 @@ func (r *BlobChunkRepository) Create(ctx context.Context, tx *sql.Tx, bc *BlobCh
 	if tx != nil {
 		_, err = tx.ExecContext(ctx, query, bc.BlobHash, bc.ChunkHash, bc.Offset, bc.Length)
 	} else {
-		_, err = r.db.conn.ExecContext(ctx, query, bc.BlobHash, bc.ChunkHash, bc.Offset, bc.Length)
+		_, err = r.db.ExecWithLock(ctx, query, bc.BlobHash, bc.ChunkHash, bc.Offset, bc.Length)
 	}
 
 	if err != nil {