feat: implement cross-batch Alipay refund reconciliation

When a refund row in an uploaded Alipay bill has no matching expense
row in the same batch (because the original purchase was uploaded in a
prior batch), the refund is now reconciled against the stored record in
bills_cleaned rather than being silently discarded.

Changes:
- analyzer/cleaners/base.py: add unresolved_refunds list to BaseCleaner
- analyzer/cleaners/alipay.py: _aggregate_refunds stores full refund
  metadata (dict); _process_expenses tracks matched keys and populates
  self.unresolved_refunds for unmatched refunds
- analyzer/server.py: thread unresolved_refunds through do_clean,
  CleanResponse, and both /clean endpoints
- server/adapter/adapter.go: add UnresolvedRefund type and field to CleanResult
- server/adapter/http/cleaner.go: deserialize unresolved_refunds from
  Python response and populate CleanResult
- server/repository/repository.go: add ReconcileRefund to BillRepository interface
- server/repository/mongo/repository.go: implement ReconcileRefund —
  full refund soft-deletes the bill, partial refund reduces amount and
  appends remark with original amount and refund order number
- server/handler/upload.go: capture clean result and call ReconcileRefund
  for each unresolved refund after saving cleaned bills
- server/model/response.go: add ReconciledRefundCount to UploadData

Also: add CLAUDE.md (@AGENTS.md), update AGENTS.md, fix DailyTrendChart
missing-date gap by filling zero-expense dates in daily map.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

server/repository/repository.go

@@ -56,4 +56,10 @@ type BillRepository interface {
 	// 用于避免京东账单与其他来源（微信、支付宝）账单重复计算
 	// 返回: 删除数量、错误
 	SoftDeleteJDRelatedBills() (int64, error)
+
+	// ReconcileRefund 将跨批次退款核销到已存储的清洗后账单
+	// 按 bill_type + (transaction_id == orderNo 或 merchant_order_no == merchantOrderNo) 查找未删除记录
+	// 全额退款（剩余金额 <= 0.005）则软删除；部分退款则扣减 amount 并追加备注
+	// 返回: 是否找到并核销了匹配记录、错误（未找到匹配记录不算错误，返回 matched=false）
+	ReconcileRefund(billType, orderNo, merchantOrderNo string, refundAmount float64, refundTime, merchant, description, refundOrderNo string) (matched bool, err error)
 }