feat(templates): remote template registry via CONTAINR_TEMPLATE_REGISTRY_URL (#52)

- GET /templates merges a remote Containr catalog as source:"registry"
  (5s timeout, 60s cache, offline-safe)
- GET /templates/:id falls through to the registry
- Deploy of a registry template materializes a deterministic private
  copy for the deployer — repeat deploys reuse it
- Templates page badges registry entries; openapi + env documented

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Co-authored-by: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

app/frontend/src/features/templates/pages/TemplatesPage.tsx

@@ -644,6 +644,11 @@ export function TemplatesPage() {
                                   Public
                                 </span>
                               ) : null}
+                              {template.source === 'registry' ? (
+                                <span className="px-1.5 py-0.5 rounded bg-[var(--accent-primary-soft)] text-[var(--accent-primary)] text-[10px] font-semibold">
+                                  Registry
+                                </span>
+                              ) : null}
                             </div>
                             <div className="flex items-center gap-2 mt-1">
                               <span