v0.4.5: Add dedicated Backup page (Biztonsági mentés)

New /backups page with full backup system visibility:
- Status overview cards (local/remote backup, DB count, repo size)
- Schedule section with next-run times and retention policy
- Database table with type, size, validation (table count), status
- Snapshot history table with per-snapshot stats
- Repository info card with paths, integrity status, remote placeholder
- "Mentés most" button with auto-refresh polling
- Empty state when backup not configured

Backend: SnapshotRecord history (ring buffer), DumpValidation,
ListDumpFiles, ListSnapshots, GetFullStatus, restic check tracking.
Server accepts scheduler for next-run time calculation.

Sidebar nav updated with 3rd item, dashboard backup card title clickable.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

controller/internal/web/templates/layout.html

@@ -16,6 +16,7 @@
         <ul class="nav-links">
             <li><a href="/" class="{{if eq .Page "dashboard"}}active{{end}}">Vezérlőpult</a></li>
             <li><a href="/stacks" class="{{if eq .Page "stacks"}}active{{end}}">Alkalmazások</a></li>
+            <li><a href="/backups" class="{{if eq .Page "backups"}}active{{end}}">Biztonsági mentés</a></li>
         </ul>
         <div class="sidebar-footer">
             <span class="version">v{{.Version}}</span>