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/repositories.go

@@ -33,6 +33,10 @@ func NewRepositories(db *DB) *Repositories {
 type TxFunc func(ctx context.Context, tx *sql.Tx) error
 
 func (r *Repositories) WithTx(ctx context.Context, fn TxFunc) error {
+	// Acquire write lock for the entire transaction
+	r.db.LockForWrite()
+	defer r.db.UnlockWrite()
+
 	tx, err := r.db.BeginTx(ctx, nil)
 	if err != nil {
 		return fmt.Errorf("beginning transaction: %w", err)