Port the Subscription resource from /api/v1 to the Huma-backed /api/v2:
POST /subscriptions/{entity}/{entityID} subscribes, DELETE unsubscribes.
The {entity} discriminator is bound as a string path param with an
enum:"project,task" tag; the model's CanCreate/CanDelete derive the numeric
EntityType from it and reject unknown kinds. Permissions and the
already-subscribed/forbidden checks come from the shared model via DoCreate/
DoDelete, identical to v1's generic handler. Mark the model's server-controlled
fields readOnly and add doc tags for the v2 schema.
kolaente