fix(app): guard force-column migration against downgrade-then-reupgrade

Review hardening for Task 8 (test-server workflow: testers install older
APKs over a newer DB):

- Old APK opening a v2 database hits drift's downgrade path, which leaves
  the force column in place but writes user_version back to 1. Reinstalling
  the new APK then re-runs onUpgrade(1,2) and addColumn crashes with
  "duplicate column name: force", failing the DB open.
- onUpgrade now checks PRAGMA table_info(upload_tasks) and only adds the
  force column when absent; the normal v1 -> v2 path is unchanged.
- Migration test simulates the downgrade-reupgrade cycle on a file-backed
  database (v2 open -> user_version forced back to 1 -> re-upgrade),
  asserting the open succeeds, legacy rows survive, and the column stays
  writable. RED before the guard (SqliteException: duplicate column name).

Co-Authored-By: Claude Code <noreply@anthropic.com>

apps/mobile/lib/features/uploads/task_store.dart

@@ -141,7 +141,20 @@ class UploadTaskDb extends _$UploadTaskDb {
   MigrationStrategy get migration => MigrationStrategy(
         onUpgrade: (m, from, to) async {
           if (from == 1) {
-            await m.addColumn(uploadTasks, uploadTasks.force);
+            // 降级-重升级防护：旧 APK 覆盖安装打开 v2 库时，drift 降级
+            // 路径不改表、只把 user_version 写回 1（force 列留存）；随后
+            // 新 APK 重升级再次走到这里——无条件 addColumn 会撞
+            // 「duplicate column name: force」致库打开失败。先探测列
+            // 存在性，缺列才补（正常 v1 → v2 路径列必缺省，行为不变）。
+            final columns = await m.database
+                .customSelect('PRAGMA table_info(upload_tasks)')
+                .get();
+            final forceExists = columns.any(
+              (row) => row.read<String>('name') == 'force',
+            );
+            if (!forceExists) {
+              await m.addColumn(uploadTasks, uploadTasks.force);
+            }
           }
         },
       );