[routes-b] GET,PATCH /api/routes-b/contacts/[id] - fetch and update a contact

[davedumto]

Summary

Implement the contacts/[id] endpoint under /api/routes-b. fetch and update a contact.

Requirements

• Create handler at app/api/routes-b/contacts/[id]/route.ts
• Implement methods: GET,PATCH
• Add request validation, auth and ownership checks, and structured error handling
• Add unit tests for the happy path and key failure modes

Acceptance criteria

• Handler implemented for each listed method at the documented path
• Validation and error envelopes match existing routes conventions
• Unit tests pass in CI