Revert mqtt_plant — go back to one shared MQTT integration

Per-app MQTT slugs meant a confusing mix of shared and per-app
credentials in one settings page. Consolidating on a single shared
identity instead — matches the original "MQTT is shared infra"
decision, and publish access stays scoped per-device regardless.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/integrations/schema.js

@@ -52,11 +52,6 @@ export const INTEGRATIONS = {
     configFields: ['host', 'port'],
     secretFields: ['username', 'password'],
   },
-  mqtt_plant: {
-    name: 'MQTT Broker (Plant)',
-    configFields: ['host', 'port'],
-    secretFields: ['username', 'password'],
-  },
 }
 
 export const MASKED = '••••••••'

src/routes/integrations.js

@@ -141,7 +141,7 @@ export async function integrationRoutes(app) {
         if (res.status === 401) throw new Error('Invalid Nextcloud credentials')
         if (!res.ok) throw new Error(`Nextcloud returned ${res.status}`)
       }
-      else if (req.params.slug === 'mqtt' || req.params.slug.startsWith('mqtt_')) {
+      else if (req.params.slug === 'mqtt') {
         if (!creds.host || !creds.username || !creds.password) throw new Error('MQTT host, username and password are required')
         const mqtt = await import('mqtt')
         await new Promise((resolve, reject) => {