feat: config history (diff/rollback) + friendly backup scheduling (v0.12.3-beta)

Configuration History:
- Per-device snapshots from the full RouterOS /export (.rsc) — config-only, so
  counters/operational state no longer cause phantom changes; deduped by content hash
- New Config History tab: snapshot timeline, readable line diff, on-demand Capture
  (with honest 'no changes' feedback), one-click rollback via the linked .rsc backup
- Snapshot and its backup are one artifact: badged 'Config Snapshot' on the Backups
  page, FK ON DELETE CASCADE keeps them in lockstep; delete is gated by a warning modal
- New config_drift alert event (off by default); periodic snapshot poll with
  configurable cadence/retention (config_snapshot_interval_min/_retention)
- New endpoints under /api/config-history (list/get/diff/capture/rollback/delete)

Scheduled Backups:
- Replaced the raw cron textbox in Settings with Frequency (Daily/Weekly/Monthly)
  + time + day pickers; cron is generated/parsed behind the scenes
- Fixed cronMatchesNow to evaluate day-of-month/month/day-of-week (it previously
  only checked minute/hour, so weekly/monthly schedules ran every day)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/src/routes/alerts.ts

@@ -20,6 +20,7 @@ const DEFAULT_RULES = [
   { event_type: 'cert_expiry',       enabled: false, threshold: 14,   cooldown_min: 1440 },
   { event_type: 'device_discovered',        enabled: false, threshold: null, cooldown_min: 60   },
   { event_type: 'firmware_update_available', enabled: true,  threshold: null, cooldown_min: 1440 },
+  { event_type: 'config_drift',      enabled: false, threshold: null, cooldown_min: 15   },
 ];
 
 async function ensureDefaultRules(): Promise<void> {