docs: add ADR-0031 for service repository structure decision

Create ADR documenting the decision to use a monorepo structure with
service directories for organizing microservices (Option 1).

The ADR covers:
- Decision to use monorepo with service directories
- Rationale for monorepo vs separate repositories
- Proposed directory structure with cmd/ and services/
- Implementation strategy across epics
- Consequences and mitigations
- Migration path if needed in the future

This clarifies how services (Auth, Identity, Authz, Audit) should be
organized in the codebase while maintaining service independence and
microservices architecture principles.

docs/content/adr/README.md

@@ -73,6 +73,7 @@ Each ADR follows this structure:
 
 - [ADR-0029: Microservices Architecture](./0029-microservices-architecture.md) - Microservices architecture from day one
 - [ADR-0030: Service Communication Strategy](./0030-service-communication-strategy.md) - Service client abstraction and communication patterns
+- [ADR-0031: Service Repository Structure](./0031-service-repository-structure.md) - Monorepo with service directories
 
 ## Adding New ADRs