feat: add Mattermost notification support

Add mmost:// and mmosts:// (secure) Apprise URL support for Mattermost
incoming webhooks. Supports optional botname override and custom paths.

- Add sendMattermost() function following existing notification patterns
- Update NotificationModal with Mattermost in examples and description

src/lib/server/notifications.ts

@@ -121,6 +121,9 @@ async function sendToAppriseUrl(url: string, payload: NotificationPayload): Prom
 			case 'slack':
 			case 'slacks':
 				return await sendSlack(url, payload);
+			case 'mmost':
+			case 'mmosts':
+				return await sendMattermost(url, payload);
 			case 'tgram':
 				return await sendTelegram(url, payload);
 			case 'gotify':
@@ -207,6 +210,54 @@ async function sendSlack(appriseUrl: string, payload: NotificationPayload): Prom
 	}
 }
 
+// Mattermost webhook
+async function sendMattermost(appriseUrl: string, payload: NotificationPayload): Promise<boolean> {
+	// mmost://[botname@]hostname[:port][/path]/token or mmosts://...
+	const isSecure = appriseUrl.startsWith('mmosts');
+	const protocol = isSecure ? 'https' : 'http';
+
+	// Remove the scheme
+	let urlPart = appriseUrl.replace(/^mmosts?:\/\//, '');
+
+	// Check for botname (username@hostname format)
+	let username: string | undefined;
+	const atIndex = urlPart.indexOf('@');
+	if (atIndex !== -1) {
+		username = urlPart.substring(0, atIndex);
+		urlPart = urlPart.substring(atIndex + 1);
+	}
+
+	// The token is the last segment, everything else is hostname[:port][/path]
+	const lastSlashIndex = urlPart.lastIndexOf('/');
+	if (lastSlashIndex === -1) {
+		console.error('[Notifications] Invalid Mattermost URL format. Expected: mmost://[botname@]hostname[:port][/path]/token');
+		return false;
+	}
+
+	const token = urlPart.substring(lastSlashIndex + 1);
+	const hostAndPath = urlPart.substring(0, lastSlashIndex);
+
+	// Build the webhook URL: {protocol}://{hostname}[:{port}][/{path}]/hooks/{token}
+	const url = `${protocol}://${hostAndPath}/hooks/${token}`;
+
+	const envTag = payload.environmentName ? ` \`${payload.environmentName}\`` : '';
+	const body: Record<string, string> = {
+		text: `*${payload.title}*${envTag}\n${payload.message}`
+	};
+
+	if (username) {
+		body.username = username;
+	}
+
+	const response = await fetch(url, {
+		method: 'POST',
+		headers: { 'Content-Type': 'application/json' },
+		body: JSON.stringify(body)
+	});
+
+	return response.ok;
+}
+
 // Telegram
 async function sendTelegram(appriseUrl: string, payload: NotificationPayload): Promise<NotificationResult> {
 	// tgram://bot_token/chat_id

src/routes/settings/notifications/NotificationModal.svelte

@@ -414,14 +414,15 @@
 							placeholder="gotify://hostname/app-token
 discord://webhook_id/webhook_token
 slack://token_a/token_b/token_c
+mmost://hostname/webhook-token
 tgram://bot_token/chat_id
 ntfy://my-topic
 pushover://user_key/api_token
 jsons://hostname/webhook/path"
-							class="flex min-h-[220px] w-full rounded-md border border-input bg-transparent px-3 py-2 text-sm shadow-sm placeholder:text-muted-foreground focus-visible:outline-none focus-visible:ring-1 focus-visible:ring-ring"
-						></textarea>
-						<p class="text-xs text-muted-foreground">
-							Supports Gotify (gotify:// or gotifys:// for HTTPS), Discord, Slack, Telegram, ntfy, Pushover, and generic JSON webhooks.
+						class="flex min-h-[220px] w-full rounded-md border border-input bg-transparent px-3 py-2 text-sm shadow-sm placeholder:text-muted-foreground focus-visible:outline-none focus-visible:ring-1 focus-visible:ring-ring"
+					></textarea>
+					<p class="text-xs text-muted-foreground">
+						Supports Gotify (gotify:// or gotifys:// for HTTPS), Discord, Slack, Mattermost (mmost:// or mmosts://), Telegram, ntfy, Pushover, and generic JSON webhooks.
 						</p>
 					</div>
 				</div>