PB-2696: Publish UserRole and release new version

[eirinikouvara]

Adds a UserRole enum to define user roles for access control within the uncertainty engine types.

Changes

• Added user_role.py: defines UserRole(str, Enum) with two roles:
  • MEMBER: the default role granted when a user is added to an org
  • ADMIN: elevated privileges; can manage members and roles
• Added test_user_role.py tests covering:
  • Member values are correct
  • Each member is a str instance (validates str, Enum behaviour)
  • Construction from a string value via UserRole("member")
  • Invalid values raise ValueError, including wrong-case inputs
  • The enum contains exactly the expected set of members

Notes
UserRole inherits from both str and Enum, meaning members can be used directly as strings (e.g. in JSON serialisation and string comparisons) without needing to access .value (similar pattern to token.py).

Behaviour	(str, Enum)	(Enum)
isinstance(UserRole.ADMIN, str)	True	False
JSON serializable by default	Yes ("admin")	No (raises TypeError)
String comparison == "admin"	True	False
.upper(), .startswith(), etc.	Works directly	AttributeError
.value needed for string ops	No	Yes

Why include the UserRole in the types library?
The UserRole enum lives in the shared types package so both the accounts service and the public core API import one canonical definition: adding or renaming a role is a single source edit, with no risk of the two services drifting out of sync over duplicated copies. (Note: only the inert role identifiers are shared this way — the role→permission mapping stays private to the accounts service.)

[eirinikouvara]

Closing as this will be moved to dev-types instead.