fix: correct BackRef type hinting

[0x054]

Description

Fix misleading IDE type hints where annotating reverse collections as BackRef[list[T]] caused .all() to appear as list[list[T]].

Changes

• Add BackRef.all() and BackRef.first() typing overloads to interpret both BackRef[T] and BackRef[list[T]] as returning model instances.
• Override BackRef.all/first to delegate to Query while preserving the improved typing surface.

Bridge and Schema Impact

• No Rust/Python bridge changes
• Python model/schema changed
• Rust core or SQL generation changed
• src/ferro/_core.pyi updated (if needed)
• Integration test added first for new behavior

Migration / Breaking Changes

• No breaking changes
• Breaking changes included (details below)

Documentation and Changelog

• No docs update needed
• Docs updated (README/docs/inline docs)
• Changelog entry needed

Related Issues

Made with Cursor