refactor: extract business logic from routes into reusable service modules

server/src/services/categoryService.ts

@@ -0,0 +1,31 @@
+import { db } from '../db/database';
+
+export function listCategories() {
+  return db.prepare('SELECT * FROM categories ORDER BY name ASC').all();
+}
+
+export function createCategory(userId: number, name: string, color?: string, icon?: string) {
+  const result = db.prepare(
+    'INSERT INTO categories (name, color, icon, user_id) VALUES (?, ?, ?, ?)'
+  ).run(name, color || '#6366f1', icon || '\uD83D\uDCCD', userId);
+  return db.prepare('SELECT * FROM categories WHERE id = ?').get(result.lastInsertRowid);
+}
+
+export function getCategoryById(categoryId: number | string) {
+  return db.prepare('SELECT * FROM categories WHERE id = ?').get(categoryId);
+}
+
+export function updateCategory(categoryId: number | string, name?: string, color?: string, icon?: string) {
+  db.prepare(`
+    UPDATE categories SET
+      name = COALESCE(?, name),
+      color = COALESCE(?, color),
+      icon = COALESCE(?, icon)
+    WHERE id = ?
+  `).run(name || null, color || null, icon || null, categoryId);
+  return db.prepare('SELECT * FROM categories WHERE id = ?').get(categoryId);
+}
+
+export function deleteCategory(categoryId: number | string) {
+  db.prepare('DELETE FROM categories WHERE id = ?').run(categoryId);
+}