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

student_db.sql

@@ -0,0 +1,24 @@
+-- student_db schema and seed data.
+
+SET SQL_MODE = "NO_AUTO_VALUE_ON_ZERO";
+START TRANSACTION;
+SET time_zone = "+00:00";
+SET NAMES utf8mb4;
+
+DROP TABLE IF EXISTS `students`;
+
+CREATE TABLE `students` (
+  `id` int NOT NULL,
+  `name` varchar(100) NOT NULL,
+  `course` varchar(50) NOT NULL,
+  `year_level` int NOT NULL,
+  PRIMARY KEY (`id`)
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_general_ci;
+
+-- Ids are a 2-digit year + a per-year counter, so these two were added in 2026.
+-- The app assigns ids, so there is no AUTO_INCREMENT here.
+INSERT INTO `students` (`id`, `name`, `course`, `year_level`) VALUES
+(26000001, 'Juan Dela Cruz', 'BSIT', 2),
+(26000002, 'Maria Santos', 'BSCS', 3);
+
+COMMIT;