controller: F7 atomic volume dumps + F6 no-single-copy + F5 stale-primary sweep (WIP, pre-build)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017CDMFpFx84pfviCTVuGGhf

controller/internal/web/templates/backups.html

@@ -12,6 +12,9 @@
 {{if .Backup}}{{if .Backup.FlashError}}
 <div class="flash flash-error">{{.Backup.FlashError}}</div>
 {{end}}{{end}}
+{{if .Backup}}{{if .Backup.SingleCopyWarning}}
+<div class="alert alert-warning">{{.Backup.SingleCopyWarning}}</div>
+{{end}}{{end}}
 
 <!-- Part B: async restore progress banner — polls /api/backup/restore-status; neutral while running,
      red only on failure (exception-color principle). Hidden until a restore op is seen. -->