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/README.md

@@ -0,0 +1,67 @@
+# Testing
+
+## Stack
+
+| Layer | Tool |
+|---|---|
+| Test runner | [Vitest](https://vitest.dev/) v4 |
+| HTTP integration | [supertest](https://github.com/ladjs/supertest) |
+| HTTP interception | [nock](https://github.com/nock/nock) (intercepts at Node http layer — works with CJS `require('axios')`) |
+| Coverage | V8 (built-in, no Babel needed) |
+
+## Running tests
+
+```bash
+# Run all tests once
+npm test
+
+# Watch mode (re-runs on file change)
+npm run test:watch
+
+# With coverage report
+npm run test:coverage
+
+# Interactive UI
+npm run test:ui
+```
+
+Coverage output lands in `coverage/` (gitignored). Open `coverage/index.html` for the HTML report.
+
+## Structure
+
+```
+tests/
+├── setup.js               # Global setup: isolated DATA_DIR, SKIP_RATE_LIMIT, console suppression
+├── unit/
+│   ├── sanitizeError.test.js  # Secret redaction patterns (API keys, tokens, passwords)
+│   ├── config.test.js         # JSON array + legacy single-instance config parsing
+│   ├── requireAuth.test.js    # Auth middleware: valid/invalid/tampered cookies
+│   ├── verifyCsrf.test.js     # CSRF double-submit cookie pattern + timing-safe compare
+│   ├── qbittorrent.test.js    # Pure utils: formatBytes, formatEta, mapTorrentToDownload
+│   └── tokenStore.test.js     # JSON file token store: store/get/clear, TTL expiry
+└── integration/
+    ├── health.test.js         # GET /health and /ready endpoints
+    └── auth.test.js           # Full login/logout/me/csrf flows via supertest + nock
+```
+
+## Key design decisions
+
+- **`server/app.js`** — Express factory extracted from `server/index.js`. Tests import `createApp()` without triggering the log-file setup, `process.exit()` calls, or background poller in the entry point.
+- **nock over `vi.mock('axios')`** — Vitest's `vi.mock` only intercepts ESM `import` statements. Since `auth.js` uses CJS `require('axios')`, nock (which patches Node's `http`/`https` modules) is the correct tool for intercepting outbound requests.
+- **`SKIP_RATE_LIMIT=1`** — All supertest requests originate from `127.0.0.1`, which would quickly exhaust per-IP rate-limit windows. Setting this env var raises the limits to `Number.MAX_SAFE_INTEGER` in both the API limiter and the login limiter.
+- **Isolated `DATA_DIR`** — Each test worker gets a unique temp directory so `tokenStore.js` file I/O never conflicts with a running dev server.
+- **`createApp({ skipRateLimits: true })`** — The app factory accepts an option to disable the general API rate limiter in addition to the env var for the login-specific limiter.
+
+## Coverage targets
+
+The tested files meet these per-file minimums (enforced in CI):
+
+| File | Lines | Branches |
+|---|---|---|
+| `server/app.js` | 85% | 65% |
+| `server/routes/auth.js` | 85% | 70% |
+| `server/middleware/requireAuth.js` | 75% | 80% |
+| `server/utils/sanitizeError.js` | 60% | — |
+| `server/utils/config.js` | 50% | 55% |
+
+`dashboard.js` and `poller.js` are large files requiring complex external-service mocks (Sonarr, Radarr, qBittorrent, Emby) and are tracked as future test coverage work.