Implement local SQLite index database with repositories

- Add SQLite database connection management with proper error handling
- Implement schema for files, chunks, blobs, and snapshots tables
- Create repository pattern for each database table
- Add transaction support with proper rollback handling
- Integrate database module with fx dependency injection
- Make index path configurable via VAULTIK_INDEX_PATH env var
- Add fatal error handling for database integrity issues
- Update DESIGN.md to clarify file_chunks vs chunk_files distinction
- Remove FinalHash from BlobInfo (blobs are content-addressable)
- Add file metadata support (mtime, ctime, mode, uid, gid, symlinks)

internal/database/file_chunks.go

@@ -0,0 +1,80 @@
+package database
+
+import (
+	"context"
+	"database/sql"
+	"fmt"
+)
+
+type FileChunkRepository struct {
+	db *DB
+}
+
+func NewFileChunkRepository(db *DB) *FileChunkRepository {
+	return &FileChunkRepository{db: db}
+}
+
+func (r *FileChunkRepository) Create(ctx context.Context, tx *sql.Tx, fc *FileChunk) error {
+	query := `
+		INSERT INTO file_chunks (path, idx, chunk_hash)
+		VALUES (?, ?, ?)
+		ON CONFLICT(path, idx) DO NOTHING
+	`
+
+	var err error
+	if tx != nil {
+		_, err = tx.ExecContext(ctx, query, fc.Path, fc.Idx, fc.ChunkHash)
+	} else {
+		_, err = r.db.conn.ExecContext(ctx, query, fc.Path, fc.Idx, fc.ChunkHash)
+	}
+
+	if err != nil {
+		return fmt.Errorf("inserting file_chunk: %w", err)
+	}
+
+	return nil
+}
+
+func (r *FileChunkRepository) GetByPath(ctx context.Context, path string) ([]*FileChunk, error) {
+	query := `
+		SELECT path, idx, chunk_hash
+		FROM file_chunks
+		WHERE path = ?
+		ORDER BY idx
+	`
+
+	rows, err := r.db.conn.QueryContext(ctx, query, path)
+	if err != nil {
+		return nil, fmt.Errorf("querying file chunks: %w", err)
+	}
+	defer CloseRows(rows)
+
+	var fileChunks []*FileChunk
+	for rows.Next() {
+		var fc FileChunk
+		err := rows.Scan(&fc.Path, &fc.Idx, &fc.ChunkHash)
+		if err != nil {
+			return nil, fmt.Errorf("scanning file chunk: %w", err)
+		}
+		fileChunks = append(fileChunks, &fc)
+	}
+
+	return fileChunks, rows.Err()
+}
+
+func (r *FileChunkRepository) DeleteByPath(ctx context.Context, tx *sql.Tx, path string) error {
+	query := `DELETE FROM file_chunks WHERE path = ?`
+
+	var err error
+	if tx != nil {
+		_, err = tx.ExecContext(ctx, query, path)
+	} else {
+		_, err = r.db.conn.ExecContext(ctx, query, path)
+	}
+
+	if err != nil {
+		return fmt.Errorf("deleting file chunks: %w", err)
+	}
+
+	return nil
+}