Add Student Management mini system

Express + MySQL CRUD API for student records, with a plain HTML frontend.

- Single-file backend (server.js) with 5 CRUD routes
- Proper HTTP status codes and a consistent error envelope
- 8-digit ids: 2-digit year + per-year 6-digit counter (e.g. 26000001),
  allocated under a MySQL advisory lock so concurrent inserts cannot collide
- student_db.sql schema and seed data
- .env for credentials; only .env.example is committed

.gitignore

@@ -0,0 +1,3 @@
+node_modules/
+.env
+scripts/