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)

internal/api/server.go

@@ -19,6 +19,8 @@ func NewServer(cfg *config.Config, database *db.DB) *fiber.App {
 	app.Use(recover.New())
 	app.Use(logger.New())
 	app.Use(cors.New())
+	RegisterSwagger(app)
+
 	app.Use(middleware.NewRateLimiter(cfg.RateLimit))
 
 	if cfg.APIKey != "" {