Filter engagement opportunities by role

[jmgasper]

Summary

• adds the scalar Prisma Role query contract used by Opportunities
• validates the role enum in the DTO
• applies the role filter to the shared Prisma where before both count and pagination
• documents the filter behavior and error contract

Validation

• focused DTO and service suites: 2 suites / 71 tests
• pnpm lint
• pnpm build
• git diff --check