[user-service] create User Service API

[davidgamez]

Describe the problem

Create the UserServiceAPI OpenAPI schema and implement only the /v1/users endpoint.

Proposed solution

Single OpenAPI schema covering users and notifications. Add scripts/api-user-service-gen.sh.

tags:
  - name: users
  - name: organizations
  - name: notifications

paths:
  /v1/users:           # GET (upsert+fetch), PUT (update profile)
  /v1/notifications/types:                # GET (list predefined types)
  /v1/notifications/subscriptions:        # GET, POST (subscriptions associated to a user)
  /v1/notifications/subscriptions/{id}:   # PATCH (toggle active), DELETE

Alternatives you've considered

No response

Additional context

No response