Run the local index in WAL mode with a busy timeout (closes #217)

The connection settings were passed as `_journal_mode=`-style
parameters, which the SQLite driver drops without an error, so the
index ran in rollback-journal mode with no busy timeout. `snapshot
list` or `info` reading during a backup could make the backup's next
write fail with "database is locked". Both open paths now pass
`_pragma=` parameters; foreign keys moved there too.

With WAL on, rows committed to the open index can still be in the
-wal file, which a copy of the main file misses. The metadata export
now copies the index with VACUUM INTO, into an empty 0600 file.

The retry after a failed open no longer claims a TRUNCATE recovery; it
retries with the same settings.

Model: opus-5-5

internal/snapshot/export_perm_test.go

@@ -1,40 +1,48 @@
-//nolint:testpackage // exercises the unexported copyFile helper
+//nolint:testpackage // exercises the unexported copyDatabase helper
 package snapshot
 
 import (
+	"context"
 	"os"
 	"path/filepath"
 	"syscall"
 	"testing"
 
 	"github.com/spf13/afero"
+	"sneak.berlin/go/vaultik/internal/database"
 )
 
-// TestCopyFileExportCopyMode verifies that the exported snapshot database
-// copy is created owner-only (0600), even under a lenient 022 umask that
-// would otherwise leave a fresh file world-readable.
+// TestCopyDatabaseExportCopyMode verifies that the exported snapshot
+// database copy is created owner-only (0600), even under a lenient 022
+// umask that would otherwise leave a fresh file world-readable.
 //
 //nolint:paralleltest // syscall.Umask is process-global; parallel tests would clash
-func TestCopyFileExportCopyMode(t *testing.T) {
+func TestCopyDatabaseExportCopyMode(t *testing.T) {
 	restore := syscall.Umask(0o022)
 	defer syscall.Umask(restore)
 
+	ctx := context.Background()
 	dir := t.TempDir()
 
 	src := filepath.Join(dir, "index.sqlite")
 
-	err := os.WriteFile(src, []byte("index data"), 0o600)
+	db, err := database.New(ctx, src)
 	if err != nil {
 		t.Fatalf("creating source index: %v", err)
 	}
 
+	err = db.Close()
+	if err != nil {
+		t.Fatalf("closing source index: %v", err)
+	}
+
 	dst := filepath.Join(dir, "snapshot.db")
 
 	sm := &SnapshotManager{fs: afero.NewOsFs()}
 
-	err = sm.copyFile(src, dst)
+	err = sm.copyDatabase(ctx, src, dst)
 	if err != nil {
-		t.Fatalf("copyFile: %v", err)
+		t.Fatalf("copyDatabase: %v", err)
 	}
 
 	info, err := os.Stat(dst)