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>

go.sum

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