fix(archiver): use serializable repo-archive queue payload (#38273)

After upgrading from 1.25.x to 1.26.x, `repo-archive` workers can fail
to unmarshal queued items:

```
Failed to unmarshal item from queue "repo-archive":
json: unable to unmarshal into Go convert.Conversion within "/Repo/Units/0/Config":
cannot derive concrete type for nil interface with finite type set
```

`ArchiveRequest` started embedding `*repo_model.Repository` in 1.26,
which does not round-trip through the JSON queue.

This change stores a minimal `archiveQueueItem` (`RepoID`, `Type`,
`CommitID`, `Paths`) in `repo-archive` and loads the repository in the
worker. `UnmarshalJSON` accepts legacy payloads that used `RepoID` or
embedded `Repo.id`.

Fixes #38272

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---------

Co-authored-by: bircni <bircni@icloud.com>

services/repository/archiver/archiver_test.go

@@ -7,7 +7,9 @@ import (
 	"testing"
 	"time"
 
+	repo_model "gitea.dev/models/repo"
 	"gitea.dev/models/unittest"
+	"gitea.dev/modules/json"
 	"gitea.dev/modules/util"
 	"gitea.dev/services/contexttest"
 
@@ -21,6 +23,22 @@ func TestMain(m *testing.M) {
 	unittest.MainTest(m)
 }
 
+func TestArchiveQueueItemJSON(t *testing.T) {
+	orig := &archiveQueueItem{
+		RepoID:              7,
+		Type:                repo_model.ArchiveZip,
+		CommitID:            "abc123",
+		Paths:               []string{"agents"},
+		ArchiveRefShortName: "main",
+	}
+	bs, err := json.Marshal(orig)
+	require.NoError(t, err)
+
+	var decoded archiveQueueItem
+	require.NoError(t, json.Unmarshal(bs, &decoded))
+	assert.Equal(t, *orig, decoded)
+}
+
 func TestArchive_Basic(t *testing.T) {
 	assert.NoError(t, unittest.PrepareTestDatabase())