Implement automated API documentation generation from OpenAPI specs

[Smartdevs17]

Description

API documentation is maintained separately and often out of date. Implement OpenAPI/Swagger spec generation from TypeScript types with automated docs deployment.

Acceptance Criteria

• OpenAPI spec generation from Zod schemas and Express routes
• API docs hosted via Swagger UI
• Automated spec update on each deployment
• Request/response example generation
• Authentication flow documentation
• SDK code generation from OpenAPI spec

Technical Scope

• backend/scripts/generate-openapi.ts
• backend/src/lib/openapi-decorators.ts
• .github/workflows/generate-docs.yml
• Edge: circular reference handling, large spec performance, versioned docs