feat(vars): project shared variables — ${{shared.KEY}} refs, CRUD, UI

- project_variables table (secret entries encrypted at rest, masked in API)
- resolveServiceEnv resolves shared.* before service lookup
- GET|PUT /projects/{id}/variables; CLI 'variables shared list|set|unset';
  MCP containr_shared_variables_{list,set}
- Settings tab → Shared Variables editor; env help text mentions shared refs

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

llms.txt

@@ -16,6 +16,7 @@ Base URL: `https://<host>/api/v1`. OpenAPI: `docs/api/openapi.yaml`.
 - **Idempotency**: send `Idempotency-Key: <uuid>` on POSTs — the first response replays for 24h; concurrent duplicates get 409. Always safe to send.
 - **Pagination**: `?limit=` + `?cursor=` (pass `pagination.next_cursor` verbatim; legacy `?page=` also works). Invalid cursors fall back to page 1.
 - **Deployments**: serialized per service — a deploy on a busy service returns `status: queued` and runs when the active one finishes. `POST /deployments/{id}/cancel` drops queued (`200 cancelled`) or aborts running (`202 cancelling`) work. `no_cache: true` on create disables the Docker layer cache. `PUT /services/{id}/variables` accepts `redeploy: true` to apply env changes to running containers.
+- **Variables**: service env values support `${{service.KEY}}` cross-service refs (builtins HOST/PORT) and `${{shared.KEY}}` project shared variables (`GET|PUT /projects/{id}/variables`, CLI `variables shared …`). Resolved at deploy/reconcile, not live-updated.
 - **Volumes**: service mounts via `PUT /services/{id}` `volumes: [{type: volume|bind, source, target, read_only}]` — replaces the whole list (`[]` clears), applied on next deploy. `GET /admin/volumes` inventories node volumes with `in_use` flags; `DELETE /admin/volumes/{name}` removes unused ones (admin).
 - **Domains**: multiple hostnames per service via `GET|POST /services/{id}/domains`, `DELETE /services/{id}/domains/{domain_id}`, `POST …/{domain_id}/default` (mirrors `services.domain`). `GET /services/{id}/domains/check` returns `ok|wrong-target|pending` per domain vs `app_settings.public_ip`/`PUBLIC_IP`. `PUT` `maintenance_mode` redirects all traffic to `GET /maintenance` (needs `base_url`/`BASE_URL` setting); `basic_auth: [{username,password}]` stores bcrypt htpasswd and gates the route — responses return usernames only.
 - **Identifiers**: all ids are UUIDs. Tokens: `cnp_` user PATs, `cagt_` agent onboarding tokens.