[BUG] InsertOnConflictDoNothing returns wrong error on Create failure

## Problem

In `collectionDb.InsertOnConflictDoNothing`, when GORM `Create` fails (`tx.Error != nil`), the code logged `zap.Error(err)` and returned `err`. The named return `err` is not assigned from the transaction in this path, so callers may receive `nil` or a stale value instead of the actual database error.

## Expected

Log and return `tx.Error` (the GORM error from the failed `Create`).