Enhance test jobs with caching, coverage, and parallel execution

CI improvements:
- Add needs: [] to run tests parallel with build (faster pipeline)
- Add pip/npm caching (faster subsequent runs)
- Add 15m timeout to prevent hung jobs
- Add pytest coverage with cobertura report for GitLab
- Add pytest JUnit report for test tab in MR
- Add vitest coverage with cobertura report for GitLab
- Add coverage regex for badge display

Frontend:
- Add @vitest/coverage-v8 dependency
- Configure vitest coverage reporter (text, cobertura, html)

frontend/vite.config.ts

@@ -16,5 +16,10 @@ export default defineConfig({
     environment: 'jsdom',
     setupFiles: './src/test/setup.ts',
     css: true,
+    coverage: {
+      provider: 'v8',
+      reporter: ['text', 'cobertura', 'html'],
+      reportsDirectory: './coverage',
+    },
   }
 })