feat: add in-memory sliding window rate limiter

tests/unit/rate-limit.test.ts

@@ -0,0 +1,25 @@
+import { describe, it, expect } from 'vitest';
+import { RateLimiter } from '$lib/server/rate-limit';
+
+describe('rate-limit', () => {
+	it('allows requests under the limit', () => {
+		const limiter = new RateLimiter(3, 60000);
+		expect(limiter.check('ip1')).toBe(true);
+		expect(limiter.check('ip1')).toBe(true);
+		expect(limiter.check('ip1')).toBe(true);
+	});
+
+	it('blocks requests over the limit', () => {
+		const limiter = new RateLimiter(2, 60000);
+		expect(limiter.check('ip1')).toBe(true);
+		expect(limiter.check('ip1')).toBe(true);
+		expect(limiter.check('ip1')).toBe(false);
+	});
+
+	it('tracks different keys independently', () => {
+		const limiter = new RateLimiter(1, 60000);
+		expect(limiter.check('ip1')).toBe(true);
+		expect(limiter.check('ip2')).toBe(true);
+		expect(limiter.check('ip1')).toBe(false);
+	});
+});