Add core server Controller base, auth hook, and buildServer

src/core/server/Controller.ts

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+import type { FastifyInstance } from 'fastify';
+
+export abstract class Controller {
+  abstract register(app: FastifyInstance): void;
+}

src/core/server/authHook.ts

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+import type { FastifyInstance, FastifyReply, FastifyRequest } from 'fastify';
+
+export function registerAuthHook(
+  app: FastifyInstance,
+  secretKey: string,
+  exemptPaths: string[] = ['/health']
+): void {
+  app.addHook('onRequest', async (request: FastifyRequest, reply: FastifyReply) => {
+    const path = request.url.split('?')[0];
+    if (!secretKey || exemptPaths.includes(path)) {
+      return;
+    }
+    const header = request.headers.authorization ?? '';
+    const token = header.startsWith('Bearer ') ? header.slice('Bearer '.length) : undefined;
+    if (token !== secretKey) {
+      reply.code(401).send({ error: 'unauthorized' });
+    }
+  });
+}

src/core/server/buildServer.ts

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+import Fastify, { type FastifyInstance } from 'fastify';
+import type { Controller } from './Controller';
+import { registerAuthHook } from './authHook';
+
+export interface BuildServerOptions {
+  secretKey: string;
+  controllers: Controller[];
+}
+
+export function buildServer(options: BuildServerOptions): FastifyInstance {
+  const app = Fastify({ logger: false });
+  registerAuthHook(app, options.secretKey);
+  for (const controller of options.controllers) {
+    controller.register(app);
+  }
+  return app;
+}

test/core/server/authHook.test.ts

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+import { describe, expect, it } from 'vitest';
+import Fastify from 'fastify';
+import { registerAuthHook } from '../../../src/core/server/authHook';
+
+function buildApp(secretKey: string) {
+  const app = Fastify();
+  registerAuthHook(app, secretKey);
+  app.get('/health', async () => ({ status: 'ok' }));
+  app.get('/recommendations', async () => ({ items: [] }));
+  return app;
+}
+
+describe('registerAuthHook', () => {
+  it('allows all requests through when secretKey is empty', async () => {
+    const app = buildApp('');
+    const res = await app.inject({ method: 'GET', url: '/recommendations' });
+    expect(res.statusCode).toBe(200);
+  });
+
+  it('always exempts /health even when a secret is configured', async () => {
+    const app = buildApp('s3cret');
+    const res = await app.inject({ method: 'GET', url: '/health' });
+    expect(res.statusCode).toBe(200);
+  });
+
+  it('rejects requests without a matching bearer token', async () => {
+    const app = buildApp('s3cret');
+    const res = await app.inject({ method: 'GET', url: '/recommendations' });
+    expect(res.statusCode).toBe(401);
+  });
+
+  it('allows requests with the correct bearer token', async () => {
+    const app = buildApp('s3cret');
+    const res = await app.inject({
+      method: 'GET',
+      url: '/recommendations',
+      headers: { authorization: 'Bearer s3cret' },
+    });
+    expect(res.statusCode).toBe(200);
+  });
+});