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/

package.json

@@ -7,6 +7,10 @@
     "dev": "nodemon server/index.js",
     "start": "node server/index.js",
     "install:all": "npm install",
+    "test": "vitest run",
+    "test:watch": "vitest",
+    "test:coverage": "vitest run --coverage",
+    "test:ui": "vitest --ui",
     "audit": "npm audit --audit-level=high",
     "audit:fix": "npm audit fix",
     "audit:critical": "npm audit --audit-level=critical"
@@ -20,8 +24,12 @@
     "helmet": "^7.0.0"
   },
   "devDependencies": {
+    "@vitest/coverage-v8": "^4.1.6",
     "concurrently": "^7.6.0",
-    "nodemon": "^3.1.14"
+    "nock": "^14.0.15",
+    "nodemon": "^3.1.14",
+    "supertest": "^7.2.2",
+    "vitest": "^4.1.6"
   },
   "keywords": [
     "sabnzbd",