Add backend unit test framework and modularize settings UI

Introduced a Vitest-based backend unit testing framework with supporting scripts, helpers, and GitHub Actions integration. Refactored the admin settings page to a modular architecture, splitting monolithic logic into feature-specific tabs and hooks for improved maintainability and testability. Updated documentation to reflect the new testing setup and settings architecture, and added new dependencies for testing utilities.

documentation/testing.md

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+# Testing
+
+**Status:** ⏳ In Progress | Backend unit testing framework (Vitest)
+
+## Overview
+Unit tests for backend logic with isolated mocks (Prisma, integrations, queue).
+
+## Key Details
+- **Runner:** Vitest (`vitest.config.ts`, Node environment)
+- **Setup:** `tests/setup.ts` sets `NODE_ENV=test`, `TZ=UTC`, blocks unmocked fetch
+- **Helpers:** `tests/helpers/prisma.ts`, `tests/helpers/job-queue.ts`
+- **GitHub Actions:** Manual workflow `.github/workflows/manual-tests.yml` runs `npm test`
+- **Coverage:** `npm run test:coverage` (reports in `coverage/`)
+- **Scope:** Backend unit tests only; no real network or services
+
+## API/Interfaces
+```
+npm run test
+npm run test:watch
+npm run test:coverage
+```
+
+## Critical Issues
+- API route unit tests are incomplete; add route-level mocks before enforcing coverage.
+
+## Related
+- [backend/services/jobs.md](backend/services/jobs.md)
+- [backend/services/scheduler.md](backend/services/scheduler.md)