Use a typed escalation fallback instead of fallbackTopic

TerdutTeam spec.escalation.fallback is now {type, address}, mirroring the
server's notification channel model. CRDs regenerated.

Claude-Session: https://claude.ai/code/session_016mBLURvJoMuUEr9cB2RpUN

internal/tdclient/client.go

@@ -208,10 +208,17 @@ type EscalationLevelRequest struct {
 	Targets        []EscalationTargetRequest `json:"targets"`
 }
 
+// NotificationTarget is a notification provider and an address it understands.
+// Both empty means none.
+type NotificationTarget struct {
+	Type    string `json:"type"`
+	Address string `json:"address"`
+}
+
 type SetEscalationRequest struct {
-	RepeatCount   int64                    `json:"repeat_count"`
-	FallbackTopic string                   `json:"fallback_topic"`
-	Levels        []EscalationLevelRequest `json:"levels"`
+	RepeatCount int64                    `json:"repeat_count"`
+	Fallback    NotificationTarget       `json:"fallback"`
+	Levels      []EscalationLevelRequest `json:"levels"`
 }
 
 // SetEscalation calls PUT /api/teams/{teamID}/escalation -- an upsert