fix: inline DB error source into DbError struct

The previous fix logged the underlying error in a separate log line,
but the error that propagated up still showed just "DbError { kind:
InsertError }" at the call site. Now the source message is captured
on the struct itself, so Debug/Display output at any call site shows
the actual Diesel error inline.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/testhelpers.rs

@@ -144,6 +144,7 @@ impl PreviewDao for TestPreviewDao {
         } else {
             Err(DbError {
                 kind: DbErrorKind::UpdateError,
+                source: None,
             })
         }
     }