Add master automation on/off switch to Settings

A single global toggle that overrides every zone's own auto_mode —
lets staff pause all HVAC automation stack-wide (e.g. during
maintenance) without touching each zone individually. Backed by the
existing config key/value store; pollOnce() short-circuits entirely
when off (no NewBook fetch, no state tracking, no device commands).

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

backend/src/db.js

@@ -146,6 +146,7 @@ export async function initDb() {
 
 async function seedDefaults() {
   const defaults = {
+    automation_enabled:     true,   // master kill switch — overrides every zone's own auto_mode
     excluded_site_ids:      [],
     poll_interval_minutes:  10,     // matches the old integration's DEFAULT_SCAN_INTERVAL
     default_arrival_time:   '15:00:00',

backend/src/lib/scheduler.js

@@ -148,6 +148,12 @@ async function saveZoneState(zoneId, roomState, bookingStatus) {
 // One poll cycle: fetch bookings once, walk every active room zone, apply heating
 // logic only where the room's state has actually changed since the last cycle.
 export async function pollOnce() {
+  // Master kill switch — overrides every zone's own auto_mode. Off means a full
+  // pause: no NewBook fetch, no state tracking, no device commands. Deliberately
+  // silent (no activity_log entry) since "paused" is a normal standing state, not
+  // an event — the Settings toggle itself is the record of when it changed.
+  if (!(await getConfig('automation_enabled', true))) return
+
   const defaults = {
     default_arrival_time: await getConfig('default_arrival_time', '15:00:00'),
     default_departure_time: await getConfig('default_departure_time', '10:00:00'),