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/cmd/controller/main.go

@@ -81,6 +81,7 @@ func main() {
 	var backupMgr *backup.Manager
 	if cfg.Backup.Enabled {
 		backupMgr = backup.NewManager(cfg, pinger, logger)
+		go backupMgr.LoadSnapshotHistory()
 	}
 
 	// --- Initialize scheduler ---
@@ -128,7 +129,7 @@ func main() {
 	apiRouter := api.NewRouter(cfg, stackMgr, syncer, cpuCollector, backupMgr, logger)
 
 	// --- Initialize web server ---
-	webServer := web.NewServer(cfg, stackMgr, cpuCollector, backupMgr, logger, Version)
+	webServer := web.NewServer(cfg, stackMgr, cpuCollector, backupMgr, sched, logger, Version)
 
 	// --- Build HTTP mux ---
 	mux := http.NewServeMux()