fix(release): separate publication time from the release date (#36761)

`published_at` was an alias for `created_at`, so a release created from
an existing tag reported that tag's commit date as its publication time,
and drafts reported one despite never having been published. It is now
stored separately, set when a release is published and null for drafts.

`created_at` in turn means the date of the commit the release points at,
matching what GitHub documents it to be, and the latest release is
selected by it again. Publishing a release for an old commit no longer
takes over the latest badge, and a tag created in the web UI is dated
the same way as one pushed from the CLI.

Fixes https://github.com/go-gitea/gitea/issues/11206
Fixes https://github.com/go-gitea/gitea/issues/38714
Fixes https://github.com/go-gitea/gitea/issues/31789

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

modules/structs/release.go

@@ -36,7 +36,7 @@ type Release struct {
 	// swagger:strfmt date-time
 	CreatedAt time.Time `json:"created_at"`
 	// swagger:strfmt date-time
-	PublishedAt time.Time `json:"published_at"`
+	PublishedAt *time.Time `json:"published_at"`
 	// The user who published the release
 	Publisher *User `json:"author"`
 	// The files attached to the release