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>

analyzer/test_jd_cleaner.py

@@ -11,7 +11,8 @@ import sys
 sys.stdout.reconfigure(encoding='utf-8')
 
 def test_jd_cleaner():
-    zip_path = r'D:\Projects\BillAI\mock_data\京东交易流水(申请时间2026年01月26日13时29分47秒)(密码683263)_209.zip'
+    base_dir = os.path.dirname(os.path.dirname(os.path.abspath(__file__)))
+    zip_path = os.path.join(base_dir, 'mock_data', '京东交易流水(申请时间2026年01月26日13时29分47秒)(密码683263)_209.zip')
     
     with zipfile.ZipFile(zip_path, 'r') as zf:
         with tempfile.TemporaryDirectory() as tmpdir: