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/handler/bills.go

@@ -2,7 +2,6 @@ package handler
 
 import (
 	"net/http"
-	"strconv"
 	"time"
 
 	"github.com/gin-gonic/gin"
@@ -154,18 +153,6 @@ func ListBills(c *gin.Context) {
 	})
 }
 
-// parsePageParam 解析分页参数
-func parsePageParam(s string, defaultVal int) int {
-	if s == "" {
-		return defaultVal
-	}
-	val, err := strconv.Atoi(s)
-	if err != nil || val < 1 {
-		return defaultVal
-	}
-	return val
-}
-
 // MonthlyStatsResponse 月度统计响应
 type MonthlyStatsResponse struct {
 	Result  bool                `json:"result"`
@@ -202,6 +189,13 @@ func MonthlyStats(c *gin.Context) {
 // ReviewStats 获取待复核数据统计
 func ReviewStats(c *gin.Context) {
 	repo := repository.GetRepository()
+	if repo == nil {
+		c.JSON(http.StatusServiceUnavailable, model.ReviewResponse{
+			Result:  false,
+			Message: "数据库未连接",
+		})
+		return
+	}
 
 	// 从MongoDB查询所有需要复核的账单
 	bills, err := repo.GetBillsNeedReview()