feat:Integration Test: DB-to-API

[Adeyemi-cmd]

Close #125

PR Summary

What changed

• Added app.ts to centralize Express app setup and routing
• Refactored index.ts to bootstrap the server from app.ts
• Added integration test file: dbApiIntegration.test.ts
• Added script: npm run test:integration

Files changed

• app.ts
• index.ts
• dbApiIntegration.test.ts
• package.json

Purpose

Verify full DB-to-API flow:

• insert a dummy PriceHistory record into PostgreSQL
• start the Express app on an ephemeral port
• call GET /api/v1/history/:asset?from=...&to=...
• assert expected JSON output

Test instructions

1. Ensure .env has DATABASE_URL and API_KEY
2. Run:
   • npm install
   • npm run test:integration

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[Sadeequ]

Resolve all conflicts please.