perf(db): enable incremental auto-vacuum so cleared data shrinks the file

lib/services/history_database.dart

@@ -796,6 +796,11 @@ class HistoryDatabase {
       await txn.delete('history_path_keys');
       await txn.delete('history');
     });
+    // Return freed pages to the OS (no-op unless the file was created with
+    // auto_vacuum enabled).
+    try {
+      await db.execute('PRAGMA incremental_vacuum');
+    } catch (_) {}
     _log.i('Cleared all history');
   }
 

lib/services/sqlite_helpers.dart

@@ -28,6 +28,10 @@ Future<Database> openAppDatabase(
       if (foreignKeys) {
         await db.execute('PRAGMA foreign_keys = ON');
       }
+      // Without auto_vacuum the file never shrinks after deletes — its size
+      // is a permanent high-water mark. Only takes effect on newly created
+      // databases; existing files keep their mode until a manual VACUUM.
+      await db.execute('PRAGMA auto_vacuum = INCREMENTAL');
       await db.rawQuery('PRAGMA journal_mode = WAL');
       await db.execute('PRAGMA synchronous = NORMAL');
     },