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/errors.go

@@ -0,0 +1,20 @@
+package database
+
+import (
+	"database/sql"
+	"fmt"
+	"os"
+)
+
+// Fatal prints an error message to stderr and exits with status 1
+func Fatal(format string, args ...interface{}) {
+	fmt.Fprintf(os.Stderr, "FATAL: "+format+"\n", args...)
+	os.Exit(1)
+}
+
+// CloseRows closes rows and exits on error
+func CloseRows(rows *sql.Rows) {
+	if err := rows.Close(); err != nil {
+		Fatal("failed to close rows: %v", err)
+	}
+}