feat: establish backend foundation

src/config/env.validation.ts

@@ -0,0 +1,46 @@
+export interface Environment {
+  DATABASE_URL: string;
+  FRONTEND_ORIGIN: string;
+  PORT: number;
+}
+
+export function validateEnv(config: Record<string, unknown>): Environment {
+  const databaseUrl = config['DATABASE_URL'];
+  const frontendOrigin = config['FRONTEND_ORIGIN'];
+  const port = config['PORT'] ?? 3000;
+
+  if (typeof databaseUrl !== 'string' || !databaseUrl.startsWith('file:')) {
+    throw new Error('DATABASE_URL must be a SQLite file URL');
+  }
+
+  if (typeof frontendOrigin !== 'string') {
+    throw new Error('FRONTEND_ORIGIN must be a valid HTTP origin');
+  }
+
+  let parsedOrigin: URL;
+  try {
+    parsedOrigin = new URL(frontendOrigin);
+  } catch {
+    throw new Error('FRONTEND_ORIGIN must be a valid HTTP origin');
+  }
+
+  if (
+    (parsedOrigin.protocol !== 'http:' && parsedOrigin.protocol !== 'https:') ||
+    parsedOrigin.pathname !== '/' ||
+    parsedOrigin.search ||
+    parsedOrigin.hash
+  ) {
+    throw new Error('FRONTEND_ORIGIN must be a valid HTTP origin');
+  }
+
+  const parsedPort = Number(port);
+  if (!Number.isInteger(parsedPort) || parsedPort < 1 || parsedPort > 65535) {
+    throw new Error('PORT must be a valid port number');
+  }
+
+  return {
+    DATABASE_URL: databaseUrl,
+    FRONTEND_ORIGIN: parsedOrigin.origin,
+    PORT: parsedPort,
+  };
+}