feat: Sprint 2 — Wursor-side pairing flow + signed plugin client

- session store + requireSession middleware (signup persists session)
- PairingService (8-char code, 5-min TTL, 5-attempt lockout, single-use, https check)
- POST /sites/pair, /sites/redeem, /sites/:id/confirm, GET /sites/:id
- PluginClient signs HMAC (timestamp+method+path+body-hash), token in Authorization
- ADR 0016; 58 api tests green

api/__tests__/lib/codes.test.ts

@@ -0,0 +1,36 @@
+import { describe, it, expect } from 'vitest';
+import { generateHmacSecret, generatePairingCode, generateToken, isHttpsUrl } from '../../src/lib/codes.ts';
+
+describe('generatePairingCode', () => {
+  it('returns an 8-char A-Z0-9 code', () => {
+    expect(generatePairingCode()).toMatch(/^[A-Z0-9]{8}$/);
+  });
+
+  it('returns distinct codes across calls', () => {
+    const seen = new Set(Array.from({ length: 50 }, () => generatePairingCode()));
+    expect(seen.size).toBeGreaterThan(40);
+  });
+});
+
+describe('generateToken / generateHmacSecret', () => {
+  it('returns a 32-byte base64url token', () => {
+    const token = generateToken();
+    expect(token).toMatch(/^[A-Za-z0-9_-]{43}$/);
+    expect(token).not.toEqual(generateToken());
+  });
+
+  it('returns a distinct hmac secret', () => {
+    expect(generateHmacSecret()).not.toEqual(generateHmacSecret());
+  });
+});
+
+describe('isHttpsUrl', () => {
+  it('accepts https URLs', () => {
+    expect(isHttpsUrl('https://example.com')).toBe(true);
+  });
+
+  it('rejects http and malformed URLs', () => {
+    expect(isHttpsUrl('http://example.com')).toBe(false);
+    expect(isHttpsUrl('not-a-url')).toBe(false);
+  });
+});