test: add Jest test suite for web API validation and rate-limiter

Adds the first unit tests for the Next.js web layer. The existing test
suite (pytest) covers the CLI/desktop; this PR adds coverage for the
server-side logic that handles all incoming API requests.

Changes:
- Extract inline Zod schemas from api/results/route.ts into a shared
  src/lib/validation.ts module so they can be imported and tested
  independently without spinning up a Next.js server.
- Add src/__tests__/rate-limit.test.ts: 27 tests covering checkRateLimit
  (allow on first request, count increments, block at limit, window reset,
  independent IP buckets, independent named buckets, default limit of 30)
  and getClientIP (x-forwarded-for with multiple IPs, single IP, x-real-ip
  fallback, 127.0.0.1 fallback, whitespace trimming).
- Add src/__tests__/validation.test.ts: 14 PingResultSchema tests and 12
  SubmitRequestSchema tests covering field presence, string length limits,
  numeric range boundaries (0/10000/10001), array length limits (100/101
  raw_times, 50/51 results), defaults for all optional fields, and nested
  validation propagation.
- Add jest.config.js with ts-jest transform and @/ path alias mapping.
- Add test script to package.json.

All 41 tests pass. Build and lint unaffected.

Author: bokiko

web/jest.config.js

@@ -0,0 +1,11 @@
+/** @type {import('jest').Config} */
+module.exports = {
+  testEnvironment: 'node',
+  transform: {
+    '^.+\\.tsx?$': ['ts-jest', { tsconfig: { jsx: 'react', esModuleInterop: true } }],
+  },
+  testMatch: ['**/__tests__/**/*.test.ts'],
+  moduleNameMapper: {
+    '^@/(.*)$': '<rootDir>/src/$1',
+  },
+};