Serve OpenAPI 3.0 spec at /openapi.v1.json (#37038)

Add a build-time conversion step that transforms the existing Swagger
2.0 spec into an OpenAPI 3.0 spec. The OAS3 spec is served alongside the
existing Swagger 2.0 spec, enabling API clients that require OAS3 to
generate code directly from Gitea's API.

This is not to be an answer to how gitea handles OAS3 long term,
but a way to use what we have to move a step forward.

---------

Signed-off-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: Claude (Opus 4.7) <noreply@anthropic.com>
Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

modules/structs/admin_user.go

@@ -30,7 +30,7 @@ type CreateUserOption struct {
 	// Whether the user has restricted access privileges
 	Restricted *bool `json:"restricted"`
 	// User visibility level: public, limited, or private
-	Visibility string `json:"visibility" binding:"In(,public,limited,private)"`
+	Visibility UserVisibility `json:"visibility" binding:"In(,public,limited,private)"`
 
 	// For explicitly setting the user creation timestamp. Useful when users are
 	// migrated from other systems. When omitted, the user's creation timestamp
@@ -79,5 +79,5 @@ type EditUserOption struct {
 	// Whether the user has restricted access privileges
 	Restricted *bool `json:"restricted"`
 	// User visibility level: public, limited, or private
-	Visibility string `json:"visibility" binding:"In(,public,limited,private)"`
+	Visibility UserVisibility `json:"visibility" binding:"In(,public,limited,private)"`
 }