feat(skills): 技能管理 - 增加可选的平台子分类策展 (#693)

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

backend/internal/database/skill_curation.go

@@ -0,0 +1,14 @@
+package database
+
+import (
+	"gorm.io/gorm"
+	"gorm.io/gorm/clause"
+	"infinite-canvas/backend/internal/model"
+)
+
+func migrateSkillCuration(tx *gorm.DB) error {
+	if err := tx.AutoMigrate(&model.SkillCurationSetting{}, &model.SkillCurationCategory{}, &model.SkillCurationAssignment{}); err != nil {
+		return err
+	}
+	return tx.Clauses(clause.OnConflict{DoNothing: true}).Create(&model.SkillCurationSetting{ID: 1}).Error
+}