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>

docs/api/openapi.yaml

@@ -7480,6 +7480,9 @@ components:
         is_public:
           type: boolean
           description: Whether a user-created template is published to the public catalog
+        source:
+          type: string
+          description: 'Catalog origin: absent for local/official templates; "registry" for entries merged from CONTAINR_TEMPLATE_REGISTRY_URL. Deploying a registry template materializes a local copy first.'
         created_at:
           type: string
           format: date-time