Add API documentation with OpenAPI/Swagger

[depadeto]

Context

The Function Manager and Smart Router expose REST APIs but there's no formal API documentation.

Task

Create an OpenAPI 3.0 spec (docs/openapi.yaml) documenting:

• POST /api/functions — create function
• GET /api/functions — list functions
• GET /api/functions/:id — get function detail
• DELETE /api/functions/:id — delete function
• POST /api/functions/:id/deploy — deploy instance
• GET /api/instances — list instances
• DELETE /api/instances/:id — remove instance

Include request/response schemas with examples.

Bonus

• Add Swagger UI served from the frontend

Reference

• control-plane/function-manager/main.go — API handlers