US7: Remove legacy auth code from propositions API and old contracts

[Mateus-Mannes]

User Story

As a backend maintainer, I need all authentication/session logic removed from propositions API so ownership is fully moved to user-service.

Scope

• Remove old auth endpoints and authentication configuration from propositions service.
• Remove obsolete identity/JWT wiring and related contracts generated for old auth routes.
• Update Swagger/security definitions to reflect new public/non-auth behavior for propositions API.
• Ensure propositions API remains intentionally public as decided.

Acceptance Criteria

• Propositions service builds and runs without legacy auth code.
• No old auth endpoints remain exposed in propositions API.
• Consumer contracts/clients are updated to the new service boundaries.

Unit Tests

• Tests covering propositions endpoints without auth coupling.
• Contract tests ensuring removed endpoints are no longer present.
• Regression tests for unchanged proposition functionality.

Local Validation

• Local smoke test confirms propositions APIs still work.
• Auth interactions are only available through user-service endpoints.