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>

web/src/lib/components/analysis/BillDetailDrawer.svelte

@@ -263,7 +263,7 @@
         {:else}
           <div>
             <div class="text-center mb-6">
-              <div class="text-3xl font-bold font-mono {record.incomeExpense === '收入' ? 'text-green-600 dark:text-green-400' : 'text-red-600 dark:text-red-400'}">
+              <div class="text-3xl font-bold font-mono {record.incomeExpense === '收入' ? 'text-green-600 dark:text-green-400' : record.incomeExpense === '不计收支' ? 'text-muted-foreground' : 'text-red-600 dark:text-red-400'}">
                 ¥{record.amount.toFixed(2)}
               </div>
               <div class="text-sm text-muted-foreground mt-1">{record.incomeExpense || '支出'}金额</div>