feat(#759): add GET /agent/:id/volunteer-linked

[nadavosa]

Description

Adds GET /agent/:id/volunteer-linked, returning all volunteers matched to any of the agent's opportunities. This is the missing data source for the NGO/agent profile's "Volunteers" section (VolunteerAgents.tsx on the frontend, currently a stub returning nothing — see need4deed-org/fe#777).

Related Issues

Closes #759

Changes

• New src/server/routes/agent/agent-volunteer.routes.ts — queries OpportunityVolunteer filtered by opportunity.agentId, with the same relations as the existing GET /opportunity/:id/volunteer-linked route
• Registered under RoutePrefix.VOLUNTEER_LINKED nested under /agent/:id, mirroring agentOpportunityRoutes
• Reuses the existing opportunityOpportunityVolunteerDTO and ApiVolunteerOpportunityGet response shape — no new SDK types needed, since this returns the same shape as the existing opportunity-scoped route
• PII masking applied via the same makePiiSerialization hook used by sibling routes

Note

Couldn't run this against a live Postgres in my working environment to smoke-test end-to-end — the query mirrors the existing, working opportunity/opportunity-volunteer.routes.ts route exactly (same relation paths, same DTO), just filtering by opportunity.agentId instead of opportunityId. Typecheck and lint are clean; full test suite passes except two pre-existing DB-connectivity sanity checks that fail without a live Postgres (unrelated to this change). Would appreciate a run against a real dev DB before merging.

Checklist

• WITHIN THE SCOPE OF AN ISSUE; No unnecessary files included
• Tests added/updated — no dedicated tests added, consistent with sibling route handlers (agent-opportunity.routes.ts, opportunity-volunteer.routes.ts), which also have no route-level test coverage
• Documentation updated
• CI passes