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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.env.example

@@ -47,6 +47,12 @@ SECRETS_KEY=
 CONTAINR_AGENT_AUTH_TOKEN=replace_with_strong_agent_secret
 # Optional rotation list (comma-separated). When set, this takes precedence.
 # CONTAINR_AGENT_AUTH_TOKENS=current_secret,next_secret
+# Bundled agent binaries dir served at /api/agents/download/:platform.
+# CONTAINR_AGENT_BIN_DIR=/agents
+# Optional remote template catalog (another Containr instance). Its
+# templates merge into GET /templates as source:"registry" and
+# materialize locally on deploy. The bundled catalog works without it.
+# CONTAINR_TEMPLATE_REGISTRY_URL=https://templates.example.com
 CORS_ORIGINS=http://localhost:3000,http://localhost:8080
 # Optional build-time API URL overrides. Leave empty for the default
 # same-origin setup - the frontend nginx proxies /api to the backend, which