[management] Refuse to pin an agent network gateway onto another account's host (#7519)

An agent network bootstrap stores its cluster as proxy_address, which selects the proxy that serves the endpoint. An account-scoped proxy only receives its own account's mappings, so a pin onto a host another account's proxy declares can never be served, and the endpoint is immutable — a dead gateway until the account deletes its settings. Nothing refused that pin; the domain unique index only arbitrates between endpoints.

Both bootstrap paths now refuse, before the insert, a host that another account's proxy declares, a host another account has labeled pins beneath (self-addressed path), or a hostname that is another account's endpoint (labeled path).

Shared clusters are unaffected: shared proxies are never foreign, and labeled pins under one cluster are never asked about, so any number of accounts still pin beneath eu.proxy.netbird.io. Registration is deliberately unchanged — refusing a proxy for another account's pin would let a pin lock a tenant out after the reaper drops its rows.

management/server/store/sql_store.go

@@ -6471,6 +6471,25 @@ func (s *SqlStore) IsClusterAddressConflicting(ctx context.Context, clusterAddre
 	return count > 0, nil
 }
 
+// HasForeignAccountProxyAtHost reports whether a proxy owned by a different
+// account declares this host. Shared proxies (account_id IS NULL) are not
+// foreign: a shared cluster is what most accounts pin their agent network
+// gateway to. The match folds case because proxies declare their address as
+// the operator spelled it while the caller's host is normalised; that costs a
+// scan of the proxies table, taken once per account when its gateway is
+// bootstrapped, not on the per-connect path IsClusterAddressConflicting serves.
+func (s *SqlStore) HasForeignAccountProxyAtHost(ctx context.Context, host, accountID string) (bool, error) {
+	var count int64
+	result := s.db.
+		Model(&proxy.Proxy{}).
+		Where("LOWER(cluster_address) = LOWER(?) AND account_id IS NOT NULL AND account_id != ?", host, accountID).
+		Count(&count)
+	if result.Error != nil {
+		return false, status.Errorf(status.Internal, "check proxy host ownership: %v", result.Error)
+	}
+	return count > 0, nil
+}
+
 func (s *SqlStore) DeleteAccountCluster(ctx context.Context, clusterAddress, accountID string) error {
 	result := s.db.
 		Where("cluster_address = ? AND account_id = ?", clusterAddress, accountID).