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/model/bill.go

@@ -88,9 +88,10 @@ type CleanedBill struct {
 	Category        string             `bson:"category" json:"category"`                   // 交易分类
 	Merchant        string             `bson:"merchant" json:"merchant"`                   // 交易对方
 	Description     string             `bson:"description" json:"description"`             // 商品说明
-	IncomeExpense   string             `bson:"income_expense" json:"income_expense"`       // 收/支
-	Amount          float64            `bson:"amount" json:"amount"`                       // 金额
-	PayMethod       string             `bson:"pay_method" json:"pay_method"`               // 支付方式
+	IncomeExpense   string             `bson:"income_expense" json:"income_expense"`                       // 收/支
+	Amount          float64            `bson:"amount" json:"amount"`                                       // 金额
+	OriginalAmount  float64            `bson:"original_amount,omitempty" json:"original_amount,omitempty"` // 原始金额（入库时），用于微信跨批次退款核销
+	PayMethod       string             `bson:"pay_method" json:"pay_method"`                               // 支付方式
 	Status          string             `bson:"status" json:"status"`                       // 交易状态
 	Remark          string             `bson:"remark" json:"remark"`                       // 备注
 	ReviewLevel     string             `bson:"review_level" json:"review_level"`           // 复核等级: HIGH/LOW/空