#### Description
- Adds a `subscription` domain: `POST`, `PUT`, and `GET
/api/v1/subscriptions`, wired with `CheckResources` + `ResourceDef`s on
the `subscription` metaresource (`create`, `list` + `update`, `read`).
Community gets a noop implementation; enterprise talks to Zeus.
- Migration `125_add_subscription_tuples` backfills the admin
subscription tuples for existing organizations.
- The legacy `/api/v1/checkout`, `/api/v1/billing`, and `/api/v1/portal`
routes are untouched; they are deleted once the frontend has moved.
#### Additional Information
Part of SigNoz/platform-pod#3091.
