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/utils/lineDiff.ts

@@ -0,0 +1,43 @@
+// Minimal, dependency-free unified line diff (LCS based). Config snapshots are
+// modest in size (hundreds of lines), so the O(n*m) table is fine here.
+
+export type DiffRowType = 'add' | 'del' | 'ctx';
+export interface DiffRow {
+  type: DiffRowType;
+  text: string;
+}
+
+export function lineDiff(oldText: string, newText: string): DiffRow[] {
+  const a = oldText.split('\n');
+  const b = newText.split('\n');
+  const n = a.length;
+  const m = b.length;
+
+  // dp[i][j] = length of LCS of a[i..] and b[j..]
+  const dp: number[][] = Array.from({ length: n + 1 }, () => new Array<number>(m + 1).fill(0));
+  for (let i = n - 1; i >= 0; i--) {
+    for (let j = m - 1; j >= 0; j--) {
+      dp[i][j] = a[i] === b[j] ? dp[i + 1][j + 1] + 1 : Math.max(dp[i + 1][j], dp[i][j + 1]);
+    }
+  }
+
+  const rows: DiffRow[] = [];
+  let i = 0;
+  let j = 0;
+  while (i < n && j < m) {
+    if (a[i] === b[j]) {
+      rows.push({ type: 'ctx', text: a[i] });
+      i++;
+      j++;
+    } else if (dp[i + 1][j] >= dp[i][j + 1]) {
+      rows.push({ type: 'del', text: a[i] });
+      i++;
+    } else {
+      rows.push({ type: 'add', text: b[j] });
+      j++;
+    }
+  }
+  while (i < n) rows.push({ type: 'del', text: a[i++] });
+  while (j < m) rows.push({ type: 'add', text: b[j++] });
+  return rows;
+}