feat(distribution): 通道费改主站承担，并按冷却期自动释放结算

利润不再扣支付通道费；站点可配置结算冷却期，到期自动入账，已释放后退款从收款站钱包回冲。同时修复当日自动批次无法追加、三方回冲打错钱包，以及测试库自增重置被 MySQL 统计缓存跳过的问题。

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/internal/model/distribution.go

@@ -18,6 +18,7 @@ type DistributionSite struct {
 	DefaultSupportGroupID *uint64   `gorm:"index" json:"default_support_group_id,omitempty"`
 	TaxEnabled            bool      `gorm:"not null;default:false" json:"tax_enabled"`
 	TaxRateBPS            int64     `gorm:"not null;default:100" json:"tax_rate_bps"`
+	SettlementHoldDays    int       `gorm:"not null;default:7" json:"settlement_hold_days"`
 	CreatedAt             time.Time `json:"created_at"`
 	UpdatedAt             time.Time `json:"updated_at"`
 }
@@ -102,6 +103,8 @@ type DistributionOrderFinancial struct {
 	SplitBaseAmountCent    int64      `gorm:"not null;default:0" json:"split_base_amount_cent"`
 	RoundingAdjustmentCent int64      `gorm:"not null;default:0" json:"rounding_adjustment_cent"`
 	SettlementStatus       string     `gorm:"size:24;not null;default:'pending';index" json:"settlement_status"`
+	ReleaseAt              *time.Time `gorm:"index:idx_distribution_financial_release,priority:2" json:"release_at,omitempty"`
+	ReleasedAt             *time.Time `json:"released_at,omitempty"`
 	SettledAt              *time.Time `json:"settled_at,omitempty"`
 	CreatedAt              time.Time  `json:"created_at"`
 	UpdatedAt              time.Time  `json:"updated_at"`