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>

frontend/src/types/index.ts

@@ -187,7 +187,7 @@ export interface Backup {
   device_name?: string;
   filename: string;
   size_bytes?: number;
-  backup_type: 'manual' | 'scheduled';
+  backup_type: 'manual' | 'scheduled' | 'config-snapshot';
   notes?: string;
   created_at: string;
 }
@@ -339,3 +339,35 @@ export interface WirelessMetricPoint {
   registered_clients?: number;
   noise_floor?: number;
 }
+
+export interface ConfigSnapshotMeta {
+  id: number;
+  device_id: number;
+  config_hash: string | null;
+  change_summary: string | null;
+  backup_id: number | null;
+  has_backup: boolean;
+  collected_at: string;
+}
+
+export interface ConfigSnapshotFull extends ConfigSnapshotMeta {
+  config_text: string | null;
+  text: string;
+}
+
+export interface ConfigDiffSide {
+  id: number;
+  text: string;
+  collected_at: string;
+}
+
+export interface ConfigDiffResponse {
+  from: ConfigDiffSide;
+  to: ConfigDiffSide;
+}
+
+export interface ConfigCaptureResult {
+  created: boolean;
+  message: string;
+  snapshot: ConfigSnapshotMeta;
+}