feat(app): 统一同步协议 — SyncModels + ApiClient.sync + SyncEngine.tryBatchSync

Flutter ↔ Rust 同步协议对齐:
- 新增 sync_models.dart: SyncReq/SyncResp/SyncChange/ConflictInfo
  与 Rust dto.rs 一一对应 (CreateJournal/UpdateJournal/DeleteJournal)
- ApiClient.sync(): 调用 POST /diary/sync 批量同步端点
- SyncEngine.tryBatchSync(): PendingOperation → SyncChange 批量提交
  成功清空队列，冲突保留待用户处理

保留原有逐个同步 trySync() 作为降级方案
后端 509/509 测试通过, Flutter analyze 0 error

app/lib/data/models/sync_models.dart

@@ -0,0 +1,176 @@
+// 同步协议模型 — 与 Rust 端 SyncReq/SyncResp/SyncChange/ConflictInfo 一一对应
+//
+// 端点: POST /api/v1/diary/sync
+// Rust DTO: crates/erp-diary/src/dto.rs (SyncReq, SyncResp, SyncChange, ConflictInfo)
+
+/// 同步请求 — 与 Rust SyncReq 对应
+///
+/// ```rust
+/// pub struct SyncReq {
+///     pub last_sync_time: Option<DateTime<Utc>>,
+///     pub changes: Vec<SyncChange>,
+/// }
+/// ```
+class SyncReq {
+  final DateTime? lastSyncTime;
+  final List<SyncChange> changes;
+
+  const SyncReq({this.lastSyncTime, this.changes = const []});
+
+  Map<String, dynamic> toJson() => {
+        if (lastSyncTime != null)
+          'last_sync_time': lastSyncTime!.toUtc().toIso8601String(),
+        'changes': changes.map((c) => c.toJson()).toList(),
+      };
+}
+
+/// 同步变更条目 — 与 Rust SyncChange 枚举对应
+///
+/// ```rust
+/// pub enum SyncChange {
+///     CreateJournal { data: serde_json::Value },
+///     UpdateJournal { id: Uuid, version: i32, data: serde_json::Value },
+///     DeleteJournal { id: Uuid, version: i32 },
+/// }
+/// ```
+sealed class SyncChange {
+  const SyncChange();
+
+  Map<String, dynamic> toJson();
+
+  /// 从 JSON 反序列化
+  factory SyncChange.fromJson(Map<String, dynamic> json) {
+    if (json.containsKey('CreateJournal')) {
+      return SyncChangeCreateJournal(
+        data: json['CreateJournal']['data'] as Map<String, dynamic>,
+      );
+    }
+    if (json.containsKey('UpdateJournal')) {
+      final inner = json['UpdateJournal'] as Map<String, dynamic>;
+      return SyncChangeUpdateJournal(
+        id: inner['id'] as String,
+        version: inner['version'] as int,
+        data: inner['data'] as Map<String, dynamic>,
+      );
+    }
+    if (json.containsKey('DeleteJournal')) {
+      final inner = json['DeleteJournal'] as Map<String, dynamic>;
+      return SyncChangeDeleteJournal(
+        id: inner['id'] as String,
+        version: inner['version'] as int,
+      );
+    }
+    throw FormatException('Unknown SyncChange variant: $json');
+  }
+}
+
+/// 创建日记变更
+class SyncChangeCreateJournal extends SyncChange {
+  final Map<String, dynamic> data;
+
+  const SyncChangeCreateJournal({required this.data});
+
+  @override
+  Map<String, dynamic> toJson() => {
+        'CreateJournal': {'data': data},
+      };
+}
+
+/// 更新日记变更
+class SyncChangeUpdateJournal extends SyncChange {
+  final String id;
+  final int version;
+  final Map<String, dynamic> data;
+
+  const SyncChangeUpdateJournal({
+    required this.id,
+    required this.version,
+    required this.data,
+  });
+
+  @override
+  Map<String, dynamic> toJson() => {
+        'UpdateJournal': {
+          'id': id,
+          'version': version,
+          'data': data,
+        },
+      };
+}
+
+/// 删除日记变更
+class SyncChangeDeleteJournal extends SyncChange {
+  final String id;
+  final int version;
+
+  const SyncChangeDeleteJournal({
+    required this.id,
+    required this.version,
+  });
+
+  @override
+  Map<String, dynamic> toJson() => {
+        'DeleteJournal': {
+          'id': id,
+          'version': version,
+        },
+      };
+}
+
+/// 同步响应 — 与 Rust SyncResp 对应
+///
+/// ```rust
+/// pub struct SyncResp {
+///     pub server_changes: Vec<serde_json::Value>,
+///     pub conflicts: Vec<ConflictInfo>,
+///     pub sync_time: DateTime<Utc>,
+/// }
+/// ```
+class SyncResp {
+  final List<Map<String, dynamic>> serverChanges;
+  final List<ConflictInfo> conflicts;
+  final DateTime syncTime;
+
+  const SyncResp({
+    required this.serverChanges,
+    required this.conflicts,
+    required this.syncTime,
+  });
+
+  factory SyncResp.fromJson(Map<String, dynamic> json) => SyncResp(
+        serverChanges: (json['server_changes'] as List)
+            .map((e) => Map<String, dynamic>.from(e as Map))
+            .toList(),
+        conflicts: (json['conflicts'] as List)
+            .map((e) => ConflictInfo.fromJson(Map<String, dynamic>.from(e as Map)))
+            .toList(),
+        syncTime: DateTime.parse(json['sync_time'] as String),
+      );
+}
+
+/// 冲突信息 — 与 Rust ConflictInfo 对应
+///
+/// ```rust
+/// pub struct ConflictInfo {
+///     pub journal_id: Uuid,
+///     pub local_version: i32,
+///     pub server_version: i32,
+/// }
+/// ```
+class ConflictInfo {
+  final String journalId;
+  final int localVersion;
+  final int serverVersion;
+
+  const ConflictInfo({
+    required this.journalId,
+    required this.localVersion,
+    required this.serverVersion,
+  });
+
+  factory ConflictInfo.fromJson(Map<String, dynamic> json) => ConflictInfo(
+        journalId: json['journal_id'] as String,
+        localVersion: json['local_version'] as int,
+        serverVersion: json['server_version'] as int,
+      );
+}