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/mongo/repository.go

@@ -4,6 +4,7 @@ package mongo
 import (
 	"context"
 	"fmt"
+	"math"
 	"time"
 
 	"go.mongodb.org/mongo-driver/bson"
@@ -16,6 +17,9 @@ import (
 	"billai-server/repository"
 )
 
+// refundEpsilon 退款核销后剩余金额的容差阈值，小于该值视为已全额退款
+const refundEpsilon = 0.005
+
 // Repository MongoDB 账单存储实现
 type Repository struct {
 	client            *mongo.Client
@@ -498,6 +502,74 @@ func (r *Repository) SoftDeleteJDRelatedBills() (int64, error) {
 	return result.ModifiedCount, nil
 }
 
+// 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 <= refundEpsilon）则软删除；部分退款则扣减 amount 并追加备注
+func (r *Repository) ReconcileRefund(billType, orderNo, merchantOrderNo string, refundAmount float64, refundTime, merchant, description, refundOrderNo string) (bool, error) {
+	if r.cleanedCollection == nil {
+		return false, fmt.Errorf("cleaned collection not initialized")
+	}
+
+	if orderNo == "" && merchantOrderNo == "" {
+		return false, nil
+	}
+
+	ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
+	defer cancel()
+
+	var or []bson.M
+	if orderNo != "" {
+		or = append(or, bson.M{"transaction_id": orderNo})
+	}
+	if merchantOrderNo != "" {
+		or = append(or, bson.M{"merchant_order_no": merchantOrderNo})
+	}
+	filter := bson.M{
+		"bill_type":  billType,
+		"is_deleted": bson.M{"$ne": true},
+		"$or":        or,
+	}
+
+	var bill model.CleanedBill
+	if err := r.cleanedCollection.FindOne(ctx, filter).Decode(&bill); err != nil {
+		if err == mongo.ErrNoDocuments {
+			return false, nil
+		}
+		return false, fmt.Errorf("查询待核销账单失败: %w", err)
+	}
+
+	remaining := bill.Amount - refundAmount
+	now := time.Now()
+
+	if remaining <= refundEpsilon {
+		update := bson.M{"$set": bson.M{
+			"is_deleted": true,
+			"updated_at": now,
+			"remark":     fmt.Sprintf("[退款核销]全额退款%.2f元(退款单号%s);%s", refundAmount, refundOrderNo, bill.Remark),
+		}}
+		_, err := r.cleanedCollection.UpdateOne(ctx, bson.M{"_id": bill.ID}, update)
+		if err != nil {
+			return false, fmt.Errorf("核销退款失败: %w", err)
+		}
+		return true, nil
+	}
+
+	remaining = math.Round(remaining*100) / 100
+	update := bson.M{"$set": bson.M{
+		"amount":     remaining,
+		"updated_at": now,
+		"remark":     fmt.Sprintf("原金额%.2f元,退款%.2f元(退款单号%s);%s", bill.Amount, refundAmount, refundOrderNo, bill.Remark),
+	}}
+	_, err := r.cleanedCollection.UpdateOne(ctx, bson.M{"_id": bill.ID}, update)
+	if err != nil {
+		return false, fmt.Errorf("核销退款失败: %w", err)
+	}
+	return true, nil
+}
+
+// 建议: 为提升 ReconcileRefund 查询性能，可为 bills_cleaned 添加索引
+// {transaction_id:1, bill_type:1} 和 {merchant_order_no:1, bill_type:1}（与现有"无索引"问题一并处理）
+
 // GetClient 获取 MongoDB 客户端（用于兼容旧代码）
 func (r *Repository) GetClient() *mongo.Client {
 	return r.client