Fix ownership cascades during account deletion

[chriscarrollsmith]

Summary

• Align ownership-style child foreign keys with ORM delete-orphan behavior using ON DELETE CASCADE metadata and an idempotent migration script.
• Delete organizations that would become userless during account deletion, while preserving shared organizations and their owned resources.
• Add regression coverage for sole-user and shared-organization deletion behavior plus direct SQL cascade behavior.

Closes #209

Test plan

• uv run pytest tests/routers/core/test_account.py tests/utils/test_models.py
• uv run python -m migrations.align_ownership_cascades .env
• uv run ty check .
• uv run ruff check routers/core/account.py utils/core/models.py utils/app/models.py migrations/align_ownership_cascades.py tests/routers/core/test_account.py tests/utils/test_models.py