feat(api): add REST API endpoints for Actions artifacts (#12140)

## Checklist

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### Tests for Go changes

- I added test coverage for Go changes...
  - [ ] in their respective `*_test.go` for unit tests.
  - [x] in the `tests/integration` directory if it involves interactions with a live Forgejo server.
- I ran...
  - [x] `make pr-go` before pushing

### Tests for JavaScript changes

(not applicable — Go-only change)

### Documentation

- [ ] I created a pull request [to the documentation](https://codeberg.org/forgejo/docs) to explain to Forgejo users how to use this change.
- [ ] I did not document these changes and I do not expect someone else to do it.

### Release notes

- [x] This change will be noticed by a Forgejo user or admin (feature, bug fix, performance, etc.). I suggest to include a release note for this change.
- [ ] This change is not visible to a Forgejo user or admin (refactor, dependency upgrade, etc.). I think there is no need to add a release note for this change.

## Summary

Add public REST API endpoints under `/api/v1/` for listing, inspecting, downloading, and deleting Actions artifacts. Previously, artifacts could only be accessed through the web UI or the internal runner API.

### New endpoints

| Method | Path | Description |
|--------|------|-------------|
| `GET` | `/repos/{owner}/{repo}/actions/artifacts` | List all artifacts for a repository |
| `GET` | `/repos/{owner}/{repo}/actions/runs/{run_id}/artifacts` | List artifacts for a workflow run |
| `GET` | `/repos/{owner}/{repo}/actions/artifacts/{artifact_id}` | Get artifact metadata |
| `GET` | `/repos/{owner}/{repo}/actions/artifacts/{artifact_id}/zip` | Download artifact as zip |
| `DELETE` | `/repos/{owner}/{repo}/actions/artifacts/{artifact_id}` | Delete an artifact |

- List endpoints support `page`, `limit`, and `name` query parameters
- Both v1-v3 (multi-file, zip on-the-fly) and v4 (single zip) artifact backends are supported
- Expired artifacts are listed with `expired: true` but cannot be downloaded
- Delete requires write permission; all other endpoints require read permission

Co-authored-by: Mathieu Fenniak <mathieu@fenniak.net>
Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/12140
Reviewed-by: Andreas Ahlenstorf <aahlenst@noreply.codeberg.org>
Reviewed-by: Mathieu Fenniak <mfenniak@noreply.codeberg.org>
Co-authored-by: ShellWen <me@shellwen.com>
Co-committed-by: ShellWen <me@shellwen.com>

modules/structs/action.go

@@ -88,3 +88,26 @@ type ListActionRunResponse struct {
 	Entries    []*ActionRun `json:"workflow_runs"`
 	TotalCount int64        `json:"total_count"`
 }
+
+// ActionArtifact represents an artifact of a workflow run
+// swagger:model
+type ActionArtifact struct {
+	// the artifact's ID
+	ID int64 `json:"id"`
+	// the artifact's name
+	Name string `json:"name"`
+	// the total size of the artifact in bytes
+	SizeInBytes int64 `json:"size_in_bytes"`
+	// the URL to download the artifact zip archive
+	ArchiveDownloadURL string `json:"archive_download_url"`
+	// whether the artifact has expired
+	Expired bool `json:"expired"`
+	// the ID of the workflow run that produced this artifact
+	RunID int64 `json:"run_id"`
+	// swagger:strfmt date-time
+	CreatedAt time.Time `json:"created_at"`
+	// swagger:strfmt date-time
+	UpdatedAt time.Time `json:"updated_at"`
+	// swagger:strfmt date-time
+	ExpiresAt time.Time `json:"expires_at"`
+}