feat(api): Add OpenAPI/Swagger documentation

[Destrayon]

Description

The REST API has no machine-readable spec. Adding OpenAPI/Swagger would enable third-party integrations, client generation, and API testing.

Acceptance Criteria

• builder.Services.AddEndpointsApiExplorer() and builder.Services.AddSwaggerGen() added
• Swagger UI available at /swagger (development only or behind auth)
• All endpoints documented with proper request/response schemas

Size Estimate

S — NuGet package + service registration + endpoint annotations

Related

None