Backend service tests use the real Upstash cache

[jenul-ferdinand]

What

• Blank the Upstash values in services.setup.ts so service tests never build an Upstash client, the way integration.setup.ts does.

Why

• cloudinary.ts calls dotenv.config() when it loads. The services setup imports every model, user.model.ts imports cloudinary.ts, and so local test runs load backend/.env.
• With the Upstash values from .env, cache.ts builds a real client. unit.service.test.ts calls fetchMostReviewed, which reads the units:popular key and, on a miss, writes the test database's units to it for a week.
• If .env holds the production credentials, a local test run can put test data in the live site's popular units list. CI has no .env, so it never hits this.

Area

backend

Notes or constraints

• Found while building Build and test pull requests in GitHub Actions #363 (PR Build and test pull requests in GitHub Actions #365).
• The model imports run before the setup file's body, so dotenv has already loaded .env by then. Assign the empty values in the body: they overwrite the loaded ones before any test file imports cache.ts. No model imports cache.ts.
• Cloudinary and Gemini keys load the same way. Blank those too unless a test needs them.