feat(gigs): add gig listing CRUD endpoints

[220nightmore-spec]

Summary

• adds a new NestJS GigModule mounted at /gigs
• supports create, list with filters, read by id, update, close, and delete flows for open gig solicitations
• adds Zod validation with standard 400 responses for invalid create/update/query payloads
• adds Swagger decorators and documentation for the new gig listing resource
• covers service behavior and HTTP behavior with unit/Supertest specs

Closes #30

Validation

• pnpm exec jest src/gigs/gig.service.spec.ts src/gigs/gig.controller.spec.ts --runInBand
• pnpm run build
• pnpm exec eslint src/app.module.ts src/gigs/*.ts --max-warnings 0
• pnpm exec jest --runInBand
• git diff --check

Note: the repository currently has many existing CRLF/Prettier findings when running the full lint:check command in this Windows checkout. The touched TypeScript files pass targeted ESLint/Prettier checks, and the full Jest suite plus TypeScript build pass.