test: add comprehensive test suite (115 tests, Vitest + supertest + nock)

Framework:
- Vitest v4 as test runner (fast ESM/CJS support, V8 coverage built-in)
- supertest for integration tests against createApp() factory
- nock for HTTP interception (works with CJS require('axios'), unlike vi.mock)

New files:
- vitest.config.js          — test config: node env, isolate, V8 coverage, per-file thresholds
- tests/setup.js             — isolated DATA_DIR per worker, SKIP_RATE_LIMIT, console suppression
- tests/README.md            — approach, structure, design decisions
- server/app.js              — testable Express factory (extracted from index.js side-effects)

Unit tests (91 tests):
- tests/unit/sanitizeError.test.js  — secret redaction: apikey, token, bearer, basic-auth URLs
- tests/unit/config.test.js         — JSON array + legacy single-instance config parsing
- tests/unit/requireAuth.test.js    — valid/invalid/tampered cookies, schema validation
- tests/unit/verifyCsrf.test.js     — double-submit pattern, timing-safe compare, safe methods
- tests/unit/qbittorrent.test.js    — formatBytes, formatEta, mapTorrentToDownload state map
- tests/unit/tokenStore.test.js     — store/get/clear lifecycle, TTL expiry, atomic disk write

Integration tests (24 tests):
- tests/integration/health.test.js  — /health and /ready endpoints
- tests/integration/auth.test.js    — full login/logout/me/csrf flows, input validation,
                                      cookie attributes, no token leakage, Emby mock via nock

Production code changes (minimal, no behaviour change):
- server/routes/auth.js: EMBY_URL captured at request-time (not module load) for testability
- server/routes/auth.js: loginLimiter max → Number.MAX_SAFE_INTEGER when SKIP_RATE_LIMIT set
- server/utils/sanitizeError.js: fix HEADER_PATTERN to redact full line (not just first token)

CI:
- .gitea/workflows/ci.yml: add parallel 'test' job (npm run test:coverage, artifact upload)
- package.json: add test/test:watch/test:coverage/test:ui scripts
- .gitignore: add coverage/

tests/unit/verifyCsrf.test.js

@@ -0,0 +1,84 @@
+/**
+ * Tests for server/middleware/verifyCsrf.js
+ *
+ * CSRF protection via the double-submit cookie pattern. These tests verify
+ * that the timing-safe comparison works correctly and that safe HTTP methods
+ * are correctly exempted.
+ */
+
+import verifyCsrf from '../../server/middleware/verifyCsrf.js';
+
+function makeReq(method, cookieToken, headerToken) {
+  return {
+    method,
+    cookies: { csrf_token: cookieToken },
+    headers: { 'x-csrf-token': headerToken }
+  };
+}
+
+function makeRes() {
+  const res = {
+    statusCode: null,
+    body: null,
+    status(code) { this.statusCode = code; return this; },
+    json(body) { this.body = body; return this; }
+  };
+  return res;
+}
+
+describe('verifyCsrf middleware', () => {
+  describe('safe methods are exempted', () => {
+    for (const method of ['GET', 'HEAD', 'OPTIONS']) {
+      it(`allows ${method} with no CSRF token`, () => {
+        const next = vi.fn();
+        verifyCsrf(makeReq(method, undefined, undefined), makeRes(), next);
+        expect(next).toHaveBeenCalledOnce();
+      });
+    }
+  });
+
+  describe('mutating methods require valid token', () => {
+    const TOKEN = 'a'.repeat(64); // 64 hex chars = 32 bytes
+
+    for (const method of ['POST', 'PUT', 'PATCH', 'DELETE']) {
+      it(`allows ${method} with matching tokens`, () => {
+        const next = vi.fn();
+        const res = makeRes();
+        verifyCsrf(makeReq(method, TOKEN, TOKEN), res, next);
+        expect(next).toHaveBeenCalledOnce();
+        expect(res.statusCode).toBeNull();
+      });
+
+      it(`blocks ${method} with mismatched tokens`, () => {
+        const next = vi.fn();
+        const res = makeRes();
+        verifyCsrf(makeReq(method, TOKEN, TOKEN.replace('a', 'b')), res, next);
+        expect(res.statusCode).toBe(403);
+        expect(next).not.toHaveBeenCalled();
+      });
+
+      it(`blocks ${method} with missing cookie token`, () => {
+        const next = vi.fn();
+        const res = makeRes();
+        verifyCsrf(makeReq(method, undefined, TOKEN), res, next);
+        expect(res.statusCode).toBe(403);
+        expect(res.body.error).toBe('CSRF token missing');
+      });
+
+      it(`blocks ${method} with missing header token`, () => {
+        const next = vi.fn();
+        const res = makeRes();
+        verifyCsrf(makeReq(method, TOKEN, undefined), res, next);
+        expect(res.statusCode).toBe(403);
+      });
+    }
+
+    it('blocks when tokens have different lengths (timing-safe path)', () => {
+      const next = vi.fn();
+      const res = makeRes();
+      verifyCsrf(makeReq('POST', 'short', 'much-longer-token-here'), res, next);
+      expect(res.statusCode).toBe(403);
+      expect(res.body.error).toBe('CSRF token invalid');
+    });
+  });
+});