[Backend] Unit and integration tests for all API routes

[Gbangbolaoluwagbemiga]

Overview

The backend has zero tests. All routes need happy-path and error-path coverage.

Setup

```bash
cd backend && npm install -D vitest supertest @types/supertest
```

Add to `backend/package.json`: `"test": "vitest run"`

Mock Supabase and Groq via `vi.mock()`.

Routes to test

Route	Happy path	Error paths
`POST /messages`	stored	invalid body, unauthorized
`GET /inbox/:address`	conversations returned	bad address, empty inbox
`POST /notifications`	created	invalid payload
`GET /notifications/:address`	list returned	not found

Acceptance criteria

• `npm test` in `backend/` passes
• ≥ 70% line coverage
• Tests run in CI