feat: implement comprehensive automated testing framework

Closes #9

What was done

Built the full automated testing framework as specified in the issue,
covering all backend modules with no real external API calls in unit tests.

Test structure

tests/
├── unit/
│ ├── nlp/parser.test.ts
│ ├── nlp/responses.test.ts
│ ├── agent/scanner.test.ts
│ ├── agent/router.test.ts
│ ├── stellar/client.test.ts
│ ├── middleware/errorHandler.test.ts
│ └── whatsapp/
│ ├── handler.test.ts
│ └── userManager.test.ts
├── integration/
│ ├── api/auth.test.ts
│ ├── api/portfolio.test.ts
│ ├── api/transactions.test.ts
│ ├── api/deposit.test.ts
│ └── whatsapp/webhook.test.ts
└── helpers/
├── testDb.ts
└── factories.ts

Results

Changes

• jest.config.ts — TypeScript Jest config with coverage thresholds and forceExit
• package.json — added test and test:coverage scripts
• .github/workflows/ci.yml — runs tests + coverage on every PR, uploads
  coverage artifact, fails build if thresholds not met
• Backend/.gitignore — added coverage/ exclusion
• src/controllers/auth-controller.ts / src/middleware/authenticate.ts —
  switched from new PrismaClient() to the shared db singleton so
  jest.mock('../db') correctly intercepts all database calls in tests,
  preventing real Postgres connections during unit tests