fix(17-06): singleton route retries a lost first-or-create race

Two first requests to GET .../singleton both missed the record; the
loser's insert hit the unique index and the route answered 500. A
unique violation from SingletonRecord now reruns the transaction once,
which finds the winner's record. TestSingleton fires six concurrent
first requests and requires 200 with the same id from each.

docs/backend/admin-controllers.md

@@ -448,7 +448,7 @@ Bulk actions have their own namespace next to the toolbar and widget actions: a
 
 ## Singleton controllers
 
-A controller that edits exactly one record, such as a site-wide definition, implements `pact.AdminSingleton`. Its `SingletonRecord(ctx, db)` returns that record and creates it first when it does not exist (first or create), with `db` the request's transaction. Make the creation safe against two first requests at once, for example with a unique key column and GORM's `FirstOrCreate`.
+A controller that edits exactly one record, such as a site-wide definition, implements `pact.AdminSingleton`. Its `SingletonRecord(ctx, db)` returns that record and creates it first when it does not exist (first or create), with `db` the request's transaction. Make the creation safe against two first requests at once, for example with a unique key column and GORM's `FirstOrCreate`: when the losing request's insert hits that unique index (a Postgres unique violation), the singleton route runs its transaction once more, and the second `SingletonRecord` call finds the winner's record.
 
 - `GET <prefix>/api/v1/{vendor}/{plugin}/{controller}/singleton` answers the record's id (`cabana.AdminSingletonResult`), after checking that the form scope (`pact.FormExtendQuery`) can load it. The admin then opens the record's form.
 - The form schema carries `singleton: true`. Show and update work as on any controller.