feat: 可选社区上报与官方精选展柜

默认关闭，仪表盘一键开关；枢纽由运维配置收报，人工精选后展示于 /showcase。

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

model/db.go

@@ -43,6 +43,7 @@ func InitDB(dbPath string) error {
 		&PointLedger{}, &CheckIn{}, &LotteryDraw{}, &PostContentUnlock{},
 		&BadgeDef{}, &UserBadge{},
 		&SitePage{}, &Poll{}, &PollOption{}, &PollVote{}, &PostLotteryWinner{},
+		&CommunityInstance{},
 	); err != nil {
 		return fmt.Errorf("自动迁移失败: %w", err)
 	}