**Backport:** https://codeberg.org/forgejo/forgejo/pulls/12351
## Fix: `GET /api/v1/repos/{owner}/{repo}/git/tags/{sha}` returns empty verification for signed tags
### Problem
When an annotated tag is signed (GPG or SSH) but the underlying commit is **not** signed, the API endpoint `GET /repos/{owner}/{repo}/git/tags/{sha}` returns an empty `verification.signature` field.
This is because `ToAnnotatedTag` was calling `ToVerification(ctx, c)` with the **commit** object, which checks the commit's signature — not the tag's own signature. Since the commit is unsigned, the API returns `signature: ""` and `verified: false`.
This causes issues for tools that rely on the tag signature from the API to validate that a tag push event is from a trusted source.
### Fix
`ToAnnotatedTag` now checks if the tag has its own signature (`t.Signature != nil`). If so, it uses `ParseTagWithSignature` to verify the tag's signature and populates the `verification` field from the tag. Otherwise, it falls back to the commit signature (existing behavior for unsigned/lightweight tags).
Co-authored-by: steven.guiheux <steven.guiheux@ovhcloud.com>
Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/12395
Reviewed-by: Mathieu Fenniak <mfenniak@noreply.codeberg.org>
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