feat: implement response envelope with success and error handling

[somotochukwu-dev]

Summary

Closes #322

Standardize hackathon API responses to a consistent { success, data, meta? } envelope across all hackathon endpoints, replacing mixed raw arrays/objects.

Changes

New utility

• src/utils/response.ts — sendSuccess(res, data, meta?, statusCode) and sendError(res, error, statusCode) helpers that produce the standard envelope. Success responses always return { success: true, data } with an optional meta key; errors return { success: false, error }.

Route migrations

Route	Before	After
GET /api (health)	{ status, timestamp, uptime, durationMs, services }	{ success: true, data: { status, timestamp, uptime, durationMs, services } }
GET /api/stats	{ success: true, data: {...} } (already wrapped)	Uses helper, no shape change
GET /api/rounds	res.json(rounds) / { source, rounds }	{ success: true, data: {...} }
GET /api/leaderboard	Raw res.json(result) with pagination at top level	{ success: true, data: { leaderboard, totalUsers, ... }, meta: { pagination } }
GET /api/prices	Raw res.json(prices)	{ success: true, data: { BTC, ETH, XLM, ... } }
POST /:id/bet, /hackathon/*/bet	{ success: true, message }	{ success: true, data: { message } }

Tests

• src/tests/hackathon-rounds.spec.ts — Updated assertions to expect res.body.data.* envelope
• src/tests/prices.routes.spec.ts — Updated assertions to expect res.body.data.* envelope
• src/tests/response-envelope.spec.ts (new) — Validates all 5 hackathon endpoints return the standard envelope

Breaking changes

Frontend consumers must update response parsers:

• Health (GET /api): response.status → response.data.status
• Rounds (GET /api/rounds): response.source → response.data.source, response.rounds → response.data.rounds
• Leaderboard (GET /api/leaderboard): response.leaderboard → response.data.leaderboard, response.pagination → response.meta.pagination
• Prices (GET /api/prices): response.BTC → response.data.BTC, response.ETH → response.data.ETH, etc.
• Bets (POST /:id/bet): response.message → response.data.message

Migration pattern for frontend:

// Before: ad-hoc per endpoint
const data = response.source ?? response.leaderboard ?? response;

// After: one parser for all
const { success, data, meta } = response;