fix: add transaction wrapping and SQL splitter documentation in migrate.ts

Each migration file now runs in a transaction - if any statement fails,
the entire file is rolled back and no _migrations record is written.
Also documents the naive semicolon-split constraint.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

server/src/migrate.ts

@@ -11,7 +11,7 @@ export async function runMigrations(pool: Pool): Promise<void> {
     CREATE TABLE IF NOT EXISTS _migrations (
       id INT UNSIGNED AUTO_INCREMENT PRIMARY KEY,
       filename VARCHAR(255) NOT NULL UNIQUE,
-      run_at DATETIME DEFAULT NOW()
+      run_at DATETIME DEFAULT CURRENT_TIMESTAMP
     )
   `);
 
@@ -28,16 +28,27 @@ export async function runMigrations(pool: Pool): Promise<void> {
     if (rows.length > 0) continue;
 
     const sql = fs.readFileSync(path.join(MIGRATIONS_DIR, file), "utf-8");
+    // NOTE: This splitter is intentionally naive — migration files must not
+    // contain semicolons inside string literals or block comments.
     const statements = sql
       .split(";")
       .map((s) => s.trim())
       .filter((s) => s.length > 0 && !s.startsWith("--"));
 
-    for (const stmt of statements) {
-      await pool.execute(stmt);
+    const conn = await pool.getConnection();
+    await conn.beginTransaction();
+    try {
+      for (const stmt of statements) {
+        await conn.execute(stmt);
+      }
+      await conn.execute("INSERT INTO _migrations (filename) VALUES (?)", [file]);
+      await conn.commit();
+      console.log(`Migration applied: ${file}`);
+    } catch (err) {
+      await conn.rollback();
+      throw err;
+    } finally {
+      conn.release();
     }
-
-    await pool.execute("INSERT INTO _migrations (filename) VALUES (?)", [file]);
-    console.log(`Migration applied: ${file}`);
   }
 }