fix(upload): 分片上传 - 先预留配额并原子提交容量 (#679)

Co-authored-by: ddcat666 <26564566+ddcat666@users.noreply.github.com>

backend/internal/model/upload_reservation.go

@@ -0,0 +1,13 @@
+package model
+
+import "time"
+
+// UploadReservation survives process restarts; expiry, not process ownership,
+// bounds its lifetime. Day records the UTC quota bucket charged at admission.
+type UploadReservation struct {
+	ID        string    `gorm:"primaryKey;size:64"`
+	UserID    string    `gorm:"index;size:64;not null"`
+	Size      int64     `gorm:"not null"`
+	Day       string    `gorm:"size:10;not null"`
+	ExpiresAt time.Time `gorm:"index;not null"`
+}