feat(db): add lightweight migration system

Introduce server-migrations.ts with a named-migration runner that tracks
applied migrations in a _migrations table. runMigrations(db) is called at
startup before routes, so additive schema changes (ALTER TABLE, new settings)
are applied once and skipped on subsequent restarts.

Update ARCHITECTURE.md: five inline edits + new §4.4 documenting the convention.

server.ts

@@ -8,6 +8,7 @@ import jwt from 'jsonwebtoken';
 import DatabaseConstructor from 'better-sqlite3';
 import { ConfidentialClientApplication } from '@azure/msal-node';
 import db, { uid, addLog, getSetting, setSetting, getAllSettings, getCaddyRoutes, addCaddyRoute, updateCaddyRoute, deleteCaddyRoute, getCaddyRouteById, DB_FILE } from './server-db';
+import { runMigrations } from './server-migrations';
 import { Device, LabTemplate, Booking, LogEntry, User, QuickLink } from './src/types';
 
 const JWT_SECRET = process.env.JWT_SECRET || 'ghostgrid-dev-secret-change-in-production';
@@ -164,6 +165,8 @@ async function pushCaddyConfig(): Promise<void> {
 }
 
 async function startServer() {
+  runMigrations(db);
+
   const app = express();
   const PORT = Number(process.env.PORT) || 3000;