Scaffold plant app - MQTT monitoring for boiler-room equipment

Read-only monitoring/alerting for boilers, water softener, calorifiers
and pumps via a generic MQTT-topic-prefix asset model, so new
equipment can be onboarded without new ingestion code. Threshold and
stale-data alert rules with email + in-app notification.

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

backend/src/routes/settings.js

@@ -0,0 +1,42 @@
+import { requireAuth, requireCap } from '../auth.js'
+import { pool } from '../db.js'
+import { isConnected as mqttConnected } from '../lib/mqtt.js'
+
+// Standard app_settings pattern (see conventions doc's "Cross-app integration
+// credentials" section) — plant only has one setting today (who gets alert
+// emails), but the shape is kept generic so a future integration is a one-row
+// addition to ALLOWED_KEYS + the seed default in db.js, not a new pattern.
+const ALLOWED_KEYS = new Set(['alert_notify_email'])
+
+export async function settingsRoutes(app) {
+  app.addHook('preHandler', requireAuth)
+
+  app.get('/api/settings', { preHandler: requireCap('settings') }, async () => {
+    const { rows } = await pool.query('SELECT key, value, updated_at FROM app_settings ORDER BY key')
+    return { settings: rows }
+  })
+
+  app.put('/api/settings', { preHandler: requireCap('settings') }, async (req, reply) => {
+    const { settings } = req.body || {}
+    if (!Array.isArray(settings)) return reply.status(400).send({ error: 'settings must be an array' })
+
+    for (const { key, value } of settings) {
+      if (!ALLOWED_KEYS.has(key)) continue
+      await pool.query(
+        `INSERT INTO app_settings (key, value, updated_at) VALUES ($1, $2, NOW())
+         ON CONFLICT (key) DO UPDATE SET value = EXCLUDED.value, updated_at = NOW()`,
+        [key, value ?? '']
+      )
+    }
+    return { ok: true }
+  })
+
+  app.get('/api/settings/mqtt-status', { preHandler: requireCap('settings') }, async () => {
+    return {
+      connected: mqttConnected(),
+      note: mqttConnected()
+        ? 'Connected to the shared MQTT broker.'
+        : 'Not connected — the shared MQTT broker (LXC 104) may be unreachable, or plant-backend has no credentials in settings.',
+    }
+  })
+}