feat: Swagger UI + OpenAPI 3.0 spec at /docs

- Full OpenAPI 3.0 spec covering all 20 endpoints
- Swagger UI served at /docs via CDN (zero deps)
- Spec at /docs/openapi.yaml (go:embed)
- Real data examples (Selic 15%, USD/BRL 5.19, etc.)
- Publicly accessible (no API key needed for docs)

README.md

@@ -16,6 +16,12 @@ The API starts on `http://localhost:3333`. On first run, it automatically fetche
 - **BCB**: Selic, CDI, IPCA, USD/BRL, EUR/BRL (last ~3 years)
 - **CVM**: Company registry + IPE filings (current + previous year)
 
+## API Documentation
+
+Visit **`/docs`** for interactive Swagger UI with full endpoint documentation.
+
+The OpenAPI 3.0 spec is also available at `/docs/openapi.yaml`.
+
 ## Endpoints
 
 | Endpoint | Description |