Give the local index and its export copy an explicit 0600 mode (closes #168)

The local index lists every backed-up path and chunk hash, but its file mode was left to the SQLite driver and the umask, so under a typical 022 umask a fresh index (and its -wal/-shm side files) landed world-readable. The snapshot export copied the index to snapshot.db with a permissive create as well.

provideDatabase now calls ensureIndexFileMode before opening the driver: it creates the index 0600 if missing and chmods an existing one to 0600. Doing this before the driver opens the file matters because SQLite gives its -wal and -shm files the mode of the main database file. The export copy is now created 0600. Tests under umask 022 cover a fresh index, an existing 0644 index, and the export copy.

Model: opus-4-8

internal/snapshot/snapshot.go

@@ -44,6 +44,7 @@ import (
 	"errors"
 	"fmt"
 	"io"
+	"os"
 	"path/filepath"
 	"strings"
 	"time"
@@ -763,7 +764,13 @@ func (sm *SnapshotManager) compressFile(inputPath, outputPath string) error {
 	return nil
 }
 
-// copyFile copies a file from src to dst
+// exportCopyPerm restricts the exported snapshot database copy to the owning
+// user; it holds the same private index data as the local index file.
+const exportCopyPerm = 0o600
+
+// copyFile copies a file from src to dst. The destination is the exported
+// snapshot database, so it is created owner-only rather than with the
+// umask-dependent default.
 func (sm *SnapshotManager) copyFile(src, dst string) error {
 	log.Debug("Opening source file for copy", "path", src)
 
@@ -783,7 +790,9 @@ func (sm *SnapshotManager) copyFile(src, dst string) error {
 
 	log.Debug("Creating destination file", "path", dst)
 
-	destFile, err := sm.fs.Create(dst)
+	destFile, err := sm.fs.OpenFile(
+		dst, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, exportCopyPerm,
+	)
 	if err != nil {
 		return err
 	}