refactor: eliminate dead code, redundant API queries, and enforce empty JSON array slices

- Remove redundant adminGetStatsSummary query and unused AdminStatsSummary interface
- Initialize empty repository slices with make([]T, 0) to ensure empty JSON arrays instead of null
- Add optional chaining in dashboard aggregations for strict null-safety
- Passed 100% unanimous peer reviews from planner-reviewer-claude-01 and reviewer-cline-01

backend/internal/repository/members.go

@@ -28,7 +28,7 @@ func (r *MembersRepository) GetMembers(ctx context.Context) ([]models.Member, er
 	}
 	defer rows.Close()
 
-	var members []models.Member
+	members := make([]models.Member, 0)
 	for rows.Next() {
 		var m models.Member
 		if err := rows.Scan(
@@ -94,7 +94,7 @@ func (r *MembersRepository) GetAlumni(ctx context.Context) ([]models.Alumnus, er
 	}
 	defer rows.Close()
 
-	var alumni []models.Alumnus
+	alumni := make([]models.Alumnus, 0)
 	for rows.Next() {
 		var a models.Alumnus
 		if err := rows.Scan(