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/agent_group.go

@@ -31,27 +31,28 @@ func NewAgentGroupService(
 }
 
 // ListAgentGroups returns paginated agent groups (handler interface method).
+//
+// SCALE-002 closure (Sprint 2, 2026-05-16): pagination is now pushed
+// into the SQL layer via AgentGroupRepository.ListPaginated, closing
+// the Bundle E / Audit L-020 "page/perPage unused" gap.
 func (s *AgentGroupService) ListAgentGroups(ctx context.Context, page, perPage int) ([]domain.AgentGroup, int64, error) {
-	// Bundle E / Audit L-020: page/perPage are unused; the underlying repo
-	// List() does not yet take pagination params. Marked explicitly so
-	// ineffassign sees no dead store and future maintainers see the
-	// vestigial params rather than a misleading default-applied clamp.
-	_ = page
-	_ = perPage
-
-	groups, err := s.groupRepo.List(ctx)
+	if page < 1 {
+		page = 1
+	}
+	if perPage < 1 {
+		perPage = 50
+	}
+	offset := (page - 1) * perPage
+	groups, total, err := s.groupRepo.ListPaginated(ctx, perPage, offset)
 	if err != nil {
 		return nil, 0, fmt.Errorf("failed to list agent groups: %w", err)
 	}
-	total := int64(len(groups))
-
 	var result []domain.AgentGroup
 	for _, g := range groups {
 		if g != nil {
 			result = append(result, *g)
 		}
 	}
-
 	return result, total, nil
 }