refactor(sync): extract stats-sync engine behind a DB-driver interface

Move snapshot/merge/quiescence engine and ssh helpers from launcher/sync
into src/core/services/stats-sync, parameterized on a minimal SyncDb
driver. The launcher binds it to bun:sqlite via launcher/sync/bun-driver;
the sync command becomes a thin adapter over the shared sync flow.

launcher/history-db.ts

@@ -43,48 +43,11 @@ export function queryLocalWatchHistory(dbPath: string): HistoryVideoRow[] {
   return withReadonlyWalRetry(dbPath, (options) => readHistoryRows(dbPath, options));
 }
 
-export function withReadonlyWalRetry<T>(
-  dbPath: string,
-  query: (options: { readonly?: boolean; readwrite?: boolean; create?: boolean }) => T,
-): T {
-  try {
-    return query({ readonly: true });
-  } catch (error) {
-    if (!isReadonlyWalRetryError(error, dbPath)) throw error;
-    return query({ readwrite: true, create: false });
-  }
-}
-
-export function isReadonlyWalRetryError(error: unknown, dbPath: string): boolean {
-  if (!isWalModeSqliteDatabase(dbPath)) return false;
-  const code =
-    typeof error === 'object' && error !== null && 'code' in error
-      ? String((error as { code?: unknown }).code ?? '')
-      : '';
-  const message = error instanceof Error ? error.message : String(error);
-  const text = `${code} ${message}`.toLowerCase();
-  return (
-    text.includes('readonly') ||
-    text.includes('read-only') ||
-    text.includes('attempt to write a readonly database') ||
-    text.includes('sqlite_cantopen') ||
-    text.includes('unable to open database file')
-  );
-}
-
-function isWalModeSqliteDatabase(dbPath: string): boolean {
-  const header = Buffer.alloc(20);
-  let fd: number | null = null;
-  try {
-    fd = fs.openSync(dbPath, 'r');
-    if (fs.readSync(fd, header, 0, header.length, 0) < header.length) return false;
-  } catch {
-    return false;
-  } finally {
-    if (fd !== null) fs.closeSync(fd);
-  }
-  return header.subarray(0, 16).toString('ascii') === 'SQLite format 3\0' && header[18] === 2;
-}
+export {
+  withReadonlyWalRetry,
+  isReadonlyWalRetryError,
+} from '../src/core/services/stats-sync/wal-retry.js';
+import { withReadonlyWalRetry } from '../src/core/services/stats-sync/wal-retry.js';
 
 function tableExists(db: Database, tableName: string): boolean {
   return Boolean(