refactor: move backend files to backend/ directory

Reorganize project structure:
- backend/cmd/openteam/ — entry point
- backend/internal/ — core packages
- backend/middleware/ — HTTP middleware
- backend/router/ — route setup
- backend/wire/ — dependency injection
- backend/pkg/ — shared utilities
- backend/go.mod, go.sum — Go module files

Updated Makefile to work from backend/ directory.
Removed old lowercase makefile.

backend/internal/dao/usage.go

@@ -0,0 +1,99 @@
+package dao
+
+import (
+	"context"
+	"opencatd-open/internal/store"
+	"time"
+
+	"gorm.io/gorm"
+	"gorm.io/gorm/clause"
+)
+
+type UsageDAO struct {
+	db *gorm.DB
+}
+
+type DailyUsageDAO struct {
+	db *gorm.DB
+}
+
+func NewUsageDAO(db *gorm.DB) *UsageDAO {
+	return &UsageDAO{db: db}
+}
+
+func NewDailyUsageDAO(db *gorm.DB) *DailyUsageDAO {
+	return &DailyUsageDAO{db: db}
+}
+
+// UsageLog DAO
+func (d *UsageDAO) Create(ctx context.Context, log *store.UsageLog) error {
+	return d.db.WithContext(ctx).Create(log).Error
+}
+
+func (d *UsageDAO) BatchCreate(ctx context.Context, logs []*store.UsageLog) error {
+	return d.db.WithContext(ctx).Create(logs).Error
+}
+
+func (d *UsageDAO) ListByUserID(ctx context.Context, userID uint64, limit, offset int) ([]*store.UsageLog, error) {
+	var logs []*store.UsageLog
+	err := d.db.WithContext(ctx).
+		Where("user_id = ?", userID).
+		Order("created_at DESC").
+		Limit(limit).
+		Offset(offset).
+		Find(&logs).Error
+	return logs, err
+}
+
+func (d *UsageDAO) Delete(ctx context.Context, id uint64) error {
+	return d.db.WithContext(ctx).Delete(&store.UsageLog{}, id).Error
+}
+
+func (d *UsageDAO) CountByUserID(ctx context.Context, userID uint64) (int64, error) {
+	var count int64
+	err := d.db.WithContext(ctx).Model(&store.UsageLog{}).Where("user_id = ?", userID).Count(&count).Error
+	return count, err
+}
+
+// UsageDaily DAO
+func (d *DailyUsageDAO) Create(ctx context.Context, log *store.UsageDaily) error {
+	return d.db.WithContext(ctx).Create(log).Error
+}
+
+func (d *DailyUsageDAO) ListByUserID(ctx context.Context, userID uint64, limit, offset int) ([]*store.UsageDaily, error) {
+	var logs []*store.UsageDaily
+	err := d.db.WithContext(ctx).
+		Where("user_id = ?", userID).
+		Order("date DESC").
+		Limit(limit).
+		Offset(offset).
+		Find(&logs).Error
+	return logs, err
+}
+
+func (d *DailyUsageDAO) GetByDate(ctx context.Context, userID uint64, date string) (*store.UsageDaily, error) {
+	var log store.UsageDaily
+	err := d.db.WithContext(ctx).
+		Where("user_id = ? AND date = ?", userID, date).
+		First(&log).Error
+	if err != nil {
+		return nil, err
+	}
+	return &log, nil
+}
+
+func (d *DailyUsageDAO) UpsertDailyUsage(ctx context.Context, log *store.UsageDaily) error {
+	return d.db.WithContext(ctx).Clauses(clause.OnConflict{
+		Columns:   []clause.Column{{Name: "user_id"}, {Name: "model_id"}, {Name: "date"}},
+		DoUpdates: clause.AssignmentColumns([]string{"requests", "input_tokens", "output_tokens", "cache_read_tokens", "cost"}),
+	}).Create(log).Error
+}
+
+func (d *DailyUsageDAO) ListByDateRange(ctx context.Context, userID uint64, start, end time.Time) ([]*store.UsageDaily, error) {
+	var logs []*store.UsageDaily
+	err := d.db.WithContext(ctx).
+		Where("user_id = ? AND date >= ? AND date <= ?", userID, start.Format("2006-01-02"), end.Format("2006-01-02")).
+		Order("date DESC").
+		Find(&logs).Error
+	return logs, err
+}