feat: implement WeChat cross-batch refund reconciliation and fix misc issues

WeChat cross-batch refund reconciliation:
- Add OriginalAmount field to CleanedBill for accurate cumulative refund math
- DeduplicateRawFile detects WeChat status-update rows (已退款/已全额退款) and
  emits WechatRefundUpdates for Go-side reconciliation (Scenario 1)
- WechatPy cleaner surfaces -退款 income rows with no same-batch expense match
  as unresolved_refunds for Go ReconcileRefund (Scenario 2)
- Add ReconcileWechatRefund to repository interface and MongoDB implementation
- upload.go step 15 iterates WechatRefundUpdates and reconciles against bills_cleaned

Bug fixes:
- ReviewStats: add nil repo check to prevent panic when DB is not connected
- JWT: remove hardcoded fallback secret; return 500/401 if JWTSecret not configured
- Remove unused parsePageParam dead code and its strconv import
- BillDetailDrawer: show 不计收支 amount in muted gray instead of red
- test_jd_cleaner.py: replace hardcoded D:\Projects\BillAI path with dynamic __file__ resolution

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

CHANGELOG.md

@@ -7,6 +7,23 @@
 
 ## [Unreleased]
 
+### 新增
+- **微信跨批次退款核销** - 重复上传微信账单时，自动识别"当前状态"字段中的退款信息并核销原始支出记录
+  - 支持「已全额退款」：原支出记录软删除
+  - 支持「已退款(¥X)」：累计退款金额从原始金额（`original_amount`）中扣减，剩余金额更新到记录
+  - 新增 `CleanedBill.original_amount` 字段，存储入库时的原始金额，保证多次部分退款累计计算正确
+  - `ReconcileWechatRefund` 接口和 MongoDB 实现
+- **支付宝跨批次退款核销** - 上传含退款行的支付宝账单时，若原消费记录来自更早的批次，自动在数据库中核销
+  - 退款行的「交易订单号」带后缀（`原订单号_退款编号`），天然幂等：重复上传时被原始数据去重拦截，不会重复核销
+  - 全额退款软删除，部分退款扣减金额并追加备注
+  - `ReconcileRefund` 接口和 MongoDB 实现
+
+### 技术改进
+- `server/service/bill.go`：`DeduplicateRawFile` 在微信账单去重阶段检测已退款状态行，收集 `WechatRefundUpdates` 供后续核销
+- `server/service/bill.go`：`saveCleanedBillsFromCSV` / `saveCleanedBillsFromJSON` 入库时同步写入 `original_amount`
+- `analyzer/cleaners/alipay.py`：`_process_expenses` 收集同批次内未匹配的退款（`unresolved_refunds`），通过 FastAPI 响应透传至 Go
+- `analyzer/cleaners/base.py`：`BaseCleaner` 基类新增 `unresolved_refunds` 属性
+
 ## [1.4.0] - 2026-03-23
 
 ### 新增