refactor(server/db): 多库(2/4)— 时间等 DB 端计算退回 Go(可移植)

auth/admin/httpapi:把 UTC_TIMESTAMP(6) 等 DB 端取值改为 Go time.Now().UTC()
作为 ? 参数传入(两库精度一致、可测、天然跨方言)。仅这三个文件不涉及
upsert/锁,故独立成提交;其余域文件的同类改动与 dialect 改动同语句交错,合入(3/4)。

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

server/internal/admin/store.go

@@ -63,8 +63,8 @@ func (s *DBStore) GetAdminByUsername(ctx context.Context, username string) (*Adm
 func (s *DBStore) CreateAdmin(ctx context.Context, username, pwHash string, totpSecretEnc []byte) (int64, error) {
 	res, err := s.db.ExecContext(ctx,
 		`INSERT INTO admins (username, pw_hash, totp_secret, status, created_at)
-		 VALUES (?, ?, ?, 'active', UTC_TIMESTAMP(6))`,
-		username, pwHash, totpSecretEnc)
+		 VALUES (?, ?, ?, 'active', ?)`,
+		username, pwHash, totpSecretEnc, time.Now().UTC())
 	if err != nil {
 		return 0, fmt.Errorf("admin.CreateAdmin: %w", err)
 	}
@@ -149,8 +149,8 @@ func (s *DBStore) WriteAudit(ctx context.Context, actor, action, target, metaJSO
 	}
 	_, err := s.db.ExecContext(ctx,
 		`INSERT INTO audit_log (actor, action, target, meta, at)
-		 VALUES (?, ?, ?, ?, UTC_TIMESTAMP(6))`,
-		actor, action, target, metaJSON)
+		 VALUES (?, ?, ?, ?, ?)`,
+		actor, action, target, metaJSON, time.Now().UTC())
 	if err != nil {
 		return fmt.Errorf("admin.WriteAudit: %w", err)
 	}