[Backend] backend/.env.test is committed with real looking secrets format

[grantfox-oss]

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Why this matters
backend/.env.test is checked into the repo to configure the Jest test environment. Even if the values are dummy test only values, committing an env file sets a bad precedent and risks a future edit accidentally introducing a real credential.

Acceptance criteria

• Audit backend/.env.test values to confirm none are real secrets
• Rename to backend/.env.test.example and load it explicitly from jest.config.ts, or clearly comment every value as a placeholder
• Document the test env setup in backend/README.md

Files to touch

• backend/.env.test
• backend/jest.config.ts
• backend/README.md

Out of scope
Rotating any credentials, none should exist here.