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>

AGENTS.md

@@ -92,6 +92,13 @@ Browser → SvelteKit proxy → Go (Gin) → handler → service → adapter →
 ### File Processing Pipeline
 Upload: ZIP/XLSX → Extract → Convert UTF-8 CSV → Detect bill type (alipay/wechat/jd) → Deduplicate against MongoDB → Clean via Python → Save cleaned data.
 
+### Cross-Batch Refund Reconciliation
+When the original purchase and its refund are in different upload batches, within-batch logic can't match them. Two reconciliation paths handle this:
+
+**Alipay** (`handler/upload.go` step 14): After cleaning, `cleaner.unresolved_refunds` (refund rows that found no matching expense in the same file) is returned by FastAPI and iterated in Go. `ReconcileRefund()` looks up the original expense by `transaction_id` or `merchant_order_no` and either soft-deletes (full refund) or deducts `amount` (partial). Alipay refund rows have their own distinct `transaction_id` (suffixed `_<refund_id>`), so they pass raw dedup and are naturally idempotent on re-upload.
+
+**WeChat** (`handler/upload.go` step 15): WeChat re-exports the *same row* (same `transaction_id`) with an updated `当前状态` field (`已全额退款` or `已退款(¥X)`). `DeduplicateRawFile` detects these duplicate rows, extracts the refund info into `DeduplicateResult.WechatRefundUpdates`, and `ReconcileWechatRefund()` applies the update. WeChat's `¥X` is the **cumulative** total refunded, so `CleanedBill.original_amount` (set at first save) is used to compute `remaining = original_amount - X`.
+
 ### Adapter (Go ↔ Python)
 `adapter.Cleaner` interface. Two modes:
 - `http` (default): calls FastAPI at `ANALYZER_URL`