Add k6 and Socket.IO load tests for API and WebSocket endpoints (#60)

[BigManly4]

closes #60

Summary

• Add k6 load test scenarios for the REST API: auth (login/register) and
  delivery CRUD + status update endpoints under /api/v1.
• Add a Socket.IO load testing harness (load-tests/socket-load) built on
  the real socket.io-client library, since neither k6 nor Artillery ship
  a maintained engine that speaks the Socket.IO protocol. It ramps up N
  concurrent simulated driver connections, authenticates each via the
  real /api/v1/auth/login endpoint, joins a delivery room, and emits
  driver_location_update on an interval, reporting connect/ack success
  rates at the end of the run.
• Add scripts/seedLoadTestData.ts, which seeds real User and Delivery
  documents through the application's own Mongoose models before a run.
  No inline mocks or hardcoded response data are used anywhere in the
  suite; every scenario exercises the real HTTP/WebSocket API against
  that seeded data.
• Both the k6 scenarios and the WebSocket harness follow a
  controller -> service -> model layering (documented in
  load-tests/README.md).

Required fix to make WebSocket testing possible

src/sockets/connectionHandler.ts (initializeSocketServer) - the module
that registers the location-tracking Socket.IO gateway - was never
attached to the HTTP server in src/server.ts. The app only ever called
app.listen(...) directly, so the WebSocket gateway was dead code with no
way to connect to it. This PR updates server.ts to use
http.createServer(app) + initializeSocketServer(httpServer), with a
graceful shutdown call to shutdownSocketServer. No behavior of the
gateway itself was changed.

Also fixed: pre-existing build failure

src/controllers/deliveryController.ts contained two unrelated modules
concatenated together (apparent bad merge), causing duplicate
Request/Response identifiers and a DeliveryStatus type mismatch that
made tsc/pnpm build fail on main independent of this PR. Split the CRUD
functions into deliveryCrudController.ts and updated the import in
routes/delivery.ts. Neither route file is mounted in routes/index.ts, so
this is a compile-only fix with no behavior change.

Known pre-existing gaps (left out of scope)

• src/routes/deliveryRoutes.ts (/eta) and src/routes/stellar.routes.ts
  are not mounted in src/routes/index.ts, so those endpoints 404 and are
  excluded from the load test scenarios.

How to run

See load-tests/README.md. In short:
cd load-tests && npm install && cp .env.example .env
npm run seed
npm run test:api:auth && npm run test:api:deliveries
npm run test:ws

Test plan

• tsc --noEmit passes for load-tests/ and the main src/ project
• eslint passes for all changed/added files
• npm run seed successfully creates driver/customer/delivery fixtures
• k6 auth and delivery scenarios complete with thresholds met
  (http_req_failed rate < 1%, p95 < 500ms)
• WebSocket harness completes with >= 95% successful connections
• Screenshot/log output of a completed run attached below

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