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/dashboard.html

@@ -82,7 +82,7 @@
 
 {{if .BackupEnabled}}
 <div class="backup-status-card">
-    <h3>Biztonsági mentés</h3>
+    <h3><a href="/backups" class="backup-card-link">Biztonsági mentés</a></h3>
     {{if .BackupStatus}}
         <div class="backup-info-row">
             <span class="backup-label">Utolsó mentés:</span>