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/publications.go

@@ -34,7 +34,7 @@ func (r *PublicationsRepository) GetPublications(ctx context.Context, category s
 	}
 	defer rows.Close()
 
-	var pubs []models.Publication
+	pubs := make([]models.Publication, 0)
 	for rows.Next() {
 		var p models.Publication
 		if err := rows.Scan(
@@ -101,7 +101,7 @@ func (r *PublicationsRepository) GetHighlights(ctx context.Context) ([]models.Pu
 	}
 	defer rows.Close()
 
-	var pubs []models.Publication
+	pubs := make([]models.Publication, 0)
 	for rows.Next() {
 		var p models.Publication
 		if err := rows.Scan(
@@ -127,7 +127,7 @@ func (r *PublicationsRepository) GetPatents(ctx context.Context) ([]models.Paten
 	}
 	defer rows.Close()
 
-	var patents []models.Patent
+	patents := make([]models.Patent, 0)
 	for rows.Next() {
 		var p models.Patent
 		if err := rows.Scan(