test: wire jest harness for QA gate (S1-f)

Add jest + ts-jest to the NestJS backend with an app.service smoke spec that
exercises Nest DI. `npm test` is the documented command the QA gate runs.

Trace: SRS §6 gate, NFR-8.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/jest.config.js

@@ -0,0 +1,16 @@
+/**
+ * Jest config for the UlHub backend (NestJS).
+ * ts-jest transpiles the TypeScript sources; decorators/metadata are honored via tsconfig.
+ * Specs live next to sources as *.spec.ts (unit) — e2e would go under test/ with a separate config.
+ */
+module.exports = {
+  moduleFileExtensions: ['js', 'json', 'ts'],
+  rootDir: 'src',
+  testRegex: '.*\\.spec\\.ts$',
+  transform: {
+    '^.+\\.ts$': ['ts-jest', { tsconfig: '<rootDir>/../tsconfig.json' }],
+  },
+  collectCoverageFrom: ['**/*.(t|j)s'],
+  coverageDirectory: '../coverage',
+  testEnvironment: 'node',
+};