[management] Decide gateway cluster ownership before the account's own view

The ownership check only ran when the account's own view of the cluster
came back empty, so an account holding any row for the host — its own
canonical proxy — skipped it entirely. A row another account left behind
under a non-canonical spelling was then never seen, and the pin the check
exists to refuse went through. That is the case worth catching, not the
one to skip: two accounts claiming one hostname is the ambiguity the
connect-time conflict check prevents going forward and cannot see for a
row written before addresses were canonicalized, and the endpoint pinned
here cannot be moved afterwards.

Ask ownership first, and narrow what counts as foreign while doing it.
The query treated a shared proxy as outside the account, which was
harmless while it ran only for a host the account had no view of — a
shared row would have given it one — but refuses the cluster most
accounts pin to once it runs first. Only a row owned by a different
account is foreign now, which is also what the name says.

management/server/store/store.go

@@ -340,7 +340,7 @@ type Store interface {
 	CountProxiesByAccountID(ctx context.Context, accountID string) (int64, error)
 	IsClusterAddressConflicting(ctx context.Context, clusterAddress, accountID string) (bool, error)
 	HasActiveProxyAtClusterAddress(ctx context.Context, clusterAddress string) (bool, error)
-	HasProxyOutsideAccountAtHost(ctx context.Context, host, accountID string) (bool, error)
+	HasForeignAccountProxyAtHost(ctx context.Context, host, accountID string) (bool, error)
 	DeleteAccountCluster(ctx context.Context, clusterAddress, accountID string) error
 
 	GetCustomDomainsCounts(ctx context.Context) (total int64, validated int64, err error)