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',