test: add duplicate user ID constraint test

tests/unit/db.test.ts

@@ -76,6 +76,29 @@ describe('Database', () => {
     });
   });
 
+  describe('User Operations', () => {
+    test('should reject duplicate user IDs', () => {
+      // First insert should succeed
+      const firstInsert = db.prepare(`
+        INSERT INTO users (id) VALUES ('34650112233')
+      `).run();
+      expect(firstInsert.changes).toBe(1);
+
+      // Second insert with same ID should fail
+      expect(() => {
+        db.prepare(`
+          INSERT INTO users (id) VALUES ('34650112233')
+        `).run();
+      }).toThrow();
+
+      // Verify only one record exists
+      const count = db.prepare(`
+        SELECT COUNT(*) as count FROM users WHERE id = '34650112233'
+      `).get();
+      expect(count.count).toBe(1);
+    });
+  });
+
   describe('Data Operations', () => {
     beforeEach(() => {
       db.exec(`