fix(repo,service): SCALE-002 — push pagination into SQL for target/issuer/team/agent_group

Sprint 2 unified-master-audit closure. Pre-fix four service List
endpoints (target, issuer, team, agent_group) called repoFoo.List(ctx)
to fetch the full table then sliced in memory:

    rows, _ := s.repo.List(ctx)
    total := int64(len(rows))
    start := (page - 1) * perPage
    end := start + perPage
    return rows[start:end], total, nil

This page-sliced in memory pattern marshals every row per request —
fine on small fleets but unacceptable for multi-tenant or large-fleet
deploys. The agent_group case was worse — the service explicitly
ignored page/perPage and returned the entire slice.

Fix:
  - New ListPaginated(ctx, limit, offset) method on each of the four
    repositories. Postgres implementations push LIMIT + OFFSET into
    the SQL plus a SELECT COUNT(*) for the total. Mirrors the cursor
    pattern already in internal/repository/postgres/certificate.go.
  - Each ListPaginated normalises limit≤0→50 and offset<0→0,
    matching the service-layer defaults that already existed.
  - Repository interfaces grow the new method so adapters stay
    swappable.
  - Service List methods now call repoFoo.ListPaginated(ctx, perPage,
    (page-1)*perPage) directly — no more memory-slice.
  - AgentGroupService.ListAgentGroups closes the Bundle E / Audit
    L-020 'page/perPage unused' gap.

Test changes:
  - sliceWindow generic helper in testutil_test.go mirrors the SQL
    LIMIT/OFFSET semantics for in-memory mocks.
  - Six mock implementers (lifecycle_test, testutil_test x2,
    agent_group_test, team_test) gain ListPaginated methods.
  - TestTeamService_List_SCALE002_PaginationPropagatesToRepo pins
    the page=2, perPage=3 → 3 rows of 10 invariant.

Closes SCALE-002.

internal/service/target.go

@@ -89,6 +89,11 @@ func NewTargetService(
 }
 
 // List returns a paginated list of deployment targets.
+//
+// SCALE-002 closure (Sprint 2, 2026-05-16): pagination is pushed into
+// the SQL layer via TargetRepository.ListPaginated. Pre-fix this called
+// targetRepo.List(ctx) and sliced in memory, which marshalled the
+// entire targets table per request — a problem on large-fleet deploys.
 func (s *TargetService) List(ctx context.Context, page, perPage int) ([]*domain.DeploymentTarget, int64, error) {
 	if page < 1 {
 		page = 1
@@ -96,21 +101,8 @@ func (s *TargetService) List(ctx context.Context, page, perPage int) ([]*domain.
 	if perPage < 1 {
 		perPage = 50
 	}
-
-	targets, err := s.targetRepo.List(ctx)
-	if err != nil {
-		return nil, 0, fmt.Errorf("failed to list targets: %w", err)
-	}
-	total := int64(len(targets))
-	start := (page - 1) * perPage
-	if start >= int(total) {
-		return nil, total, nil
-	}
-	end := start + perPage
-	if end > int(total) {
-		end = int(total)
-	}
-	return targets[start:end], total, nil
+	offset := (page - 1) * perPage
+	return s.targetRepo.ListPaginated(ctx, perPage, offset)
 }
 
 // Get retrieves a deployment target by ID.