feat: GET /agent/:id/volunteer-linked — volunteers connected to an agent via its opportunities

[nadavosa]

Context

Linked to need4deed-org/fe#777 — the NGO/agent profile's "Volunteers" section (VolunteerAgents.tsx) is currently a stub with no data source at all, so it always shows 0 across every status tab even when the agent has volunteers matched via its opportunities.

Problem

There's no endpoint that returns "all volunteers connected to this agent through its opportunities." The equivalent already exists per-opportunity (GET /opportunity/:id/volunteer-linked), and per-agent for opportunities (GET /agent/:id/opportunity-linked), but not the agent→volunteer aggregate.

Proposed solution

Add GET /agent/:id/volunteer-linked, mirroring agent-opportunity.routes.ts / opportunity-volunteer.routes.ts:

• Query OpportunityVolunteer filtered by opportunity.agentId = :id (join through the opportunity relation), with the same relations as the existing opportunity-scoped route (volunteer.person, volunteer.deal.dealActivity.activity, volunteer.deal.dealSkill.skill, volunteer.deal.dealLanguage.language, volunteer.deal.dealTimeslot.timeslot, volunteer.deal.dealDistrict.district).
• Reuse the existing opportunityOpportunityVolunteerDTO (services/dto/dto-opportunity-volunteer.ts) — no new SDK types needed, response shape is the same ApiVolunteerOpportunityGet[] the opportunity-scoped route already returns.
• Register under RoutePrefix.VOLUNTEER_LINKED (already exists) nested under /agent/:id, same pattern as agentOpportunityRoutes.
• Apply the same makePiiSerialization PII masking as the sibling routes.

Acceptance criteria

• GET /agent/:id/volunteer-linked returns all volunteers matched to any of the agent's opportunities, in the same shape as GET /opportunity/:id/volunteer-linked
• PII masking applied consistently with sibling routes
• Agents with no matched volunteers return an empty array (not an error)