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>

server/repository/repository.go

@@ -62,4 +62,10 @@ type BillRepository interface {
 	// 全额退款（剩余金额 <= 0.005）则软删除；部分退款则扣减 amount 并追加备注
 	// 返回: 是否找到并核销了匹配记录、错误（未找到匹配记录不算错误，返回 matched=false）
 	ReconcileRefund(billType, orderNo, merchantOrderNo string, refundAmount float64, refundTime, merchant, description, refundOrderNo string) (matched bool, err error)
+
+	// ReconcileWechatRefund 将跨批次微信退款核销到已存储的清洗后账单
+	// 微信退款通过重复上传时"当前状态"字段携带退款信息来触发
+	// fullRefund=true 时软删除原记录；否则用 original_amount - cumulativeRefundAmount 计算剩余金额
+	// 返回: 是否找到并核销了匹配记录、错误
+	ReconcileWechatRefund(transactionID string, fullRefund bool, cumulativeRefundAmount float64) (matched bool, err error)
 }