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);
+            }
           }
         },
       );

apps/mobile/test/features/uploads/task_store_test.dart

@@ -11,6 +11,8 @@
 /// - datetime 文本落库的亚秒精度。
 library;
 
+import 'dart:io';
+
 import 'package:drift/drift.dart' show driftRuntimeOptions;
 import 'package:drift/native.dart' show NativeDatabase;
 import 'package:flutter_test/flutter_test.dart';
@@ -558,46 +560,52 @@ void main() {
   });
 
   group('schema 迁移（v1 → v2：force 列，Task 8）', () {
+    /// v1 底库种子（建表无 force 列 + legacy 行 + user_version=1）。语句
+    /// 拆列供内存/文件两种打开方式复用（raw 类型由 setup 上下文推断，
+    /// 免引 sqlite3 传递依赖）。
+    const List<String> v1SeedStatements = <String>[
+      '''CREATE TABLE IF NOT EXISTS "upload_tasks" (
+        "id" TEXT NOT NULL PRIMARY KEY,
+        "account_id" TEXT NOT NULL,
+        "album_id" TEXT NOT NULL,
+        "category_id" TEXT NULL,
+        "client_task_id" TEXT NOT NULL,
+        "source_file_path" TEXT NOT NULL,
+        "filename" TEXT NOT NULL,
+        "bytes" INTEGER NOT NULL,
+        "content_type" TEXT NOT NULL,
+        "state" TEXT NOT NULL,
+        "attempt" INTEGER NOT NULL DEFAULT 0,
+        "next_retry_at" TEXT NULL,
+        "server_upload_id" TEXT NULL,
+        "server_photo_id" TEXT NULL,
+        "last_error" TEXT NULL,
+        "cleaned_at" TEXT NULL,
+        "created_at" TEXT NOT NULL,
+        "updated_at" TEXT NOT NULL
+      );''',
+      '''INSERT INTO upload_tasks (
+        "id", "account_id", "album_id", "client_task_id",
+        "source_file_path", "filename", "bytes", "content_type",
+        "state", "created_at", "updated_at"
+      ) VALUES (
+        'task-legacy', 'alice', 'alb-1', 'ct-legacy',
+        '/managed/uploads/alb-1/legacy.jpg', 'IMG_LEGACY.jpg', 2048,
+        'image/jpeg', 'completed',
+        '2026-09-01T00:00:00.000Z', '2026-09-01T00:01:00.000Z'
+      );''',
+      'PRAGMA user_version = 1;',
+    ];
+
+    void runSeed(raw) {
+      for (final statement in v1SeedStatements) {
+        raw.execute(statement);
+      }
+    }
+
     /// v1 底库（无 force 列）+ 一条既有行，user_version=1——升级后打开。
-    UploadTaskDb legacyV1Db() => UploadTaskDb(
-          NativeDatabase.memory(setup: (raw) {
-            raw.execute('''
-              CREATE TABLE IF NOT EXISTS "upload_tasks" (
-                "id" TEXT NOT NULL PRIMARY KEY,
-                "account_id" TEXT NOT NULL,
-                "album_id" TEXT NOT NULL,
-                "category_id" TEXT NULL,
-                "client_task_id" TEXT NOT NULL,
-                "source_file_path" TEXT NOT NULL,
-                "filename" TEXT NOT NULL,
-                "bytes" INTEGER NOT NULL,
-                "content_type" TEXT NOT NULL,
-                "state" TEXT NOT NULL,
-                "attempt" INTEGER NOT NULL DEFAULT 0,
-                "next_retry_at" TEXT NULL,
-                "server_upload_id" TEXT NULL,
-                "server_photo_id" TEXT NULL,
-                "last_error" TEXT NULL,
-                "cleaned_at" TEXT NULL,
-                "created_at" TEXT NOT NULL,
-                "updated_at" TEXT NOT NULL
-              );
-            ''');
-            raw.execute('''
-              INSERT INTO upload_tasks (
-                "id", "account_id", "album_id", "client_task_id",
-                "source_file_path", "filename", "bytes", "content_type",
-                "state", "created_at", "updated_at"
-              ) VALUES (
-                'task-legacy', 'alice', 'alb-1', 'ct-legacy',
-                '/managed/uploads/alb-1/legacy.jpg', 'IMG_LEGACY.jpg', 2048,
-                'image/jpeg', 'completed',
-                '2026-09-01T00:00:00.000Z', '2026-09-01T00:01:00.000Z'
-              );
-            ''');
-            raw.execute('PRAGMA user_version = 1;');
-          }),
-        );
+    UploadTaskDb legacyV1Db() =>
+        UploadTaskDb(NativeDatabase.memory(setup: runSeed));
 
     test('旧库升级：既有行保留、force=false 默认、新列可写', () async {
       final db = legacyV1Db();
@@ -617,5 +625,49 @@ void main() {
       );
       expect(created.force, isTrue);
     });
+
+    test('降级重开后再升级：force 列已存在不重复补列（旧 APK 覆盖安装防护）',
+        () async {
+      final tmp = await Directory.systemTemp.createTemp('store-mig-');
+      addTearDown(() async {
+        try {
+          await tmp.delete(recursive: true);
+        } on Object {
+          // Windows 句柄延迟释放：残留交由系统临时目录清理。
+        }
+      });
+      final dbFile = File('${tmp.path}/upload_tasks.sqlite');
+
+      // ① 新 APK 打开 v1 底库：升级补 force 列（user_version → 2）。
+      final first = TaskStore(
+        UploadTaskDb(NativeDatabase(dbFile, setup: runSeed)),
+      );
+      expect((await first.get('alice', 'task-legacy'))!.force, isFalse,
+          reason: '前置：v1 → v2 升级成功');
+      await first.close();
+
+      // ② 模拟旧 APK 覆盖安装后的降级打开残留态：drift 降级路径不删列、
+      //    只把 user_version 写回 1（force 列留存）——setup 直接置 1。
+      // ③ 再装回新 APK：from=1 重跑 onUpgrade——不得因列已存在抛
+      //    「duplicate column name: force」致库打不开（测试服务器工作流：
+      //    测试员旧包覆盖装新包的常态路径）。
+      final second = TaskStore(
+        UploadTaskDb(
+          NativeDatabase(
+            dbFile,
+            setup: (raw) => raw.execute('PRAGMA user_version = 1;'),
+          ),
+        ),
+      );
+      addTearDown(second.close);
+
+      final legacy = await second.get('alice', 'task-legacy');
+      expect(legacy, isNotNull, reason: '降级-重升级循环不丢既有行');
+      expect(legacy!.filename, 'IMG_LEGACY.jpg');
+      final created = await second.create(
+        draft(id: 'task-after', clientTaskId: 'ct-after', force: true),
+      );
+      expect(created.force, isTrue, reason: '重升级后新列仍可写');
+    });
   });
 }