Centralize the *_no_tags / *_no_ext / *_extension columns (derived from a
file name) behind @validates hooks instead of computing them by hand at
every write site:
- Add pure helpers (compute_file_name_parts and friends) to models.base;
the filesystem base handler now delegates to them.
- Add @validates on Rom (fs_name), BaseAsset (file_name, inherited by all
asset subclasses), and Firmware (file_name).
- update_rom keeps the fs_name-derived columns in sync on bulk update(),
which also fixes the rename path never updating fs_extension.
- Drop the now-redundant computations at the scan/rename call sites.
Also fix the migration backfill loop and a pre-existing list[str | None]
type mismatch surfaced in scan_handler. Add tests for the helpers, the
validators, and the update_rom bulk-sync path.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
