fix: P0+P1 critical bug fixes across controller (24 files)

Concurrency fixes:
- Deep-copy stacks in GetStack/GetStacks to prevent shared state mutation (C04)
- Add per-state mutex to watchdog pathProbeState (C05)
- Guard MetricsCollector.Start() with sync.Once against double-start (C06)
- Hold diskJobMu across entire raw mount operation (C07)
- Add mutex to SetEncryptionKey (C08), MigrateEncryption write lock (H03)
- Use sync.Once for sync.Stop() channel close (H08)
- Set syncing=true before releasing lock in TriggerSync (H09)
- Deep-copy lastDBDump/lastBackup in GetFullStatus (H11)
- Add WaitGroup for stderr goroutine in MigrateDrive (H19)
- Add mutex to SetBackupRunningCheck (M18)

Security fixes:
- Validate Bearer token against Hub API key in CSRF middleware (H16)
- Validate backup paths start with expected prefix in RemoveStack (M12)
- Guard uuid[:8] slice with length check (H20)
- Parse fstab fields exactly for mount target matching (H21)

Bug fixes:
- Use decrypted env vars for compose deploy (C01)
- Log decrypt failures in DecryptMap instead of swallowing (C02)
- Move Deployed=false inside lock in runComposeDeploy (C03)
- Fix activeDrives() to skip disconnected drives (H02)
- Fix Snapshot() stderr extraction from exec.ExitError (H01)
- Check unlockCmd.Run() error in restic (H01)
- Buffer template rendering via bytes.Buffer (H07)
- Thread context.Context through cloudflare client (H10)
- Fix leaf-name collision detection in cross-drive backup (H15)
- Add nil check for crossDriveRunner (H17)
- Use strings.TrimSpace instead of slice on command output (H18)
- Make SaveAppConfig atomic with write-to-tmp+rename (H04)
- Pass encKey on deploy failure SaveAppConfig (H05)
- Fix IPv6 address format in TCP health probe

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

controller/internal/sync/sync.go

@@ -30,6 +30,7 @@ type Syncer struct {
 	lastErr      error
 	syncing      bool
 	stopCh       chan struct{}
+	stopOnce     sync.Once
 }
 
 // SyncStatus holds information about the last sync operation.
@@ -110,9 +111,11 @@ func (s *Syncer) Start() {
 	}()
 }
 
-// Stop terminates the periodic sync loop.
+// Stop terminates the periodic sync loop. Safe to call multiple times.
 func (s *Syncer) Stop() {
-	close(s.stopCh)
+	s.stopOnce.Do(func() {
+		close(s.stopCh)
+	})
 }
 
 // TriggerSync performs an immediate sync. Returns the result.
@@ -131,6 +134,7 @@ func (s *Syncer) TriggerSync() SyncResult {
 		s.mu.Unlock()
 		return SyncResult{OK: false, Message: "Túl gyakori szinkronizálás — várj 30 másodpercet"}
 	}
+	s.syncing = true
 	s.mu.Unlock()
 
 	return s.doSync()