Manage notification channels instead of an ntfy topic

Mirrors the server's channels API: add/remove channels per user from the Users
section. Requires the matching server release.

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

internal/api/client.go

@@ -623,21 +623,32 @@ func (c *Client) CreateUser(username, email string) (*User, error) {
 	return &user, c.do(req, &user)
 }
 
-// SetUserNotifyTarget points a user's push notifications at an ntfy topic.
+// AddChannel gives a user a notification channel: a provider type the server
+// has configured ("ntfy") and an address that provider understands (a topic).
 //
-// An empty topic clears it: the server stores NULL, and that user's incidents
-// page the shared fallback topic instead — which carries no Acknowledge button,
-// because anyone subscribed to it could otherwise acknowledge as somebody else.
-func (c *Client) SetUserNotifyTarget(userID int64, topic string) (*User, error) {
+// With no channel a user's incidents page the shared fallback instead, which
+// carries no Acknowledge button, because anyone subscribed to it could
+// otherwise acknowledge as somebody else.
+func (c *Client) AddChannel(userID int64, typ, address string) (*NotificationChannel, error) {
 	body := struct {
-		NtfyTopic string `json:"ntfy_topic"`
-	}{NtfyTopic: topic}
-	req, err := c.newRequestWithBody(http.MethodPut, fmt.Sprintf("/api/users/%d/notify", userID), body)
+		Type    string `json:"type"`
+		Address string `json:"address"`
+	}{Type: typ, Address: address}
+	req, err := c.newRequestWithBody(http.MethodPost, fmt.Sprintf("/api/users/%d/channels", userID), body)
 	if err != nil {
 		return nil, err
 	}
-	var user User
-	return &user, c.do(req, &user)
+	var ch NotificationChannel
+	return &ch, c.do(req, &ch)
+}
+
+// RemoveChannel deletes one of a user's notification channels.
+func (c *Client) RemoveChannel(userID, channelID int64) error {
+	req, err := c.newRequest(http.MethodDelete, fmt.Sprintf("/api/users/%d/channels/%d", userID, channelID))
+	if err != nil {
+		return err
+	}
+	return c.do(req, nil)
 }
 
 func (c *Client) DeleteUser(id int64) error {