fix(migrations): preserve unique constraints in v334 sync (#37743)

The truncated `ActionRunner` struct in
`AddCancellingSupportToActionRunner` declares only the new
`HasCancellingSupport` column. When xorm's `SyncWithOptions` compares it
against the live `action_runner` table, every index/constraint absent
from the local struct is a candidate for removal.

Walking [xorm v1.3.11
sync.go:250-266](https://gitea.com/xorm/xorm/src/tag/v1.3.11/sync.go#L250-L266):

- `IndexType` indices skip the drop when `IgnoreIndices ||
IgnoreDropIndices` — already covered.
- `UniqueType` indices skip the drop only when `IgnoreConstrains` —
**not** set in #37275, so the existing `UNIQUE` on `token_hash` (and any
other uniques) would be dropped on upgrade.

Adding `IgnoreConstrains: true` matches v333's pattern and preserves the
existing unique constraints. Spotted by @wxiaoguang in
https://github.com/go-gitea/gitea/pull/37275#discussion_r3254168680.

---
This PR was written with the help of Claude Opus 4.7

Co-authored-by: Claude (Opus 4.7) <noreply@anthropic.com>
Co-authored-by: Nicolas <bircni@icloud.com>

models/migrations/v1_27/v334.go

@@ -11,6 +11,7 @@ func AddCancellingSupportToActionRunner(x *xorm.Engine) error {
 	}
 
 	_, err := x.SyncWithOptions(xorm.SyncOptions{
+		IgnoreConstrains:  true,
 		IgnoreDropIndices: true,
 	}, new(ActionRunner))
 	return err