Column diff, detect missing columns and type/attribute changes

## Describe the feature

Compare parsed GORM model fields with database column metadata and emit add_column, modify_column (length, precision/scale, unsigned, nullability, default, auto increment, collation), drop_column.

## Motivation

- Eliminate schema drift by comparing GORM models to live database metadata and surfacing add_column/modify_column deltas.
- Improve safety: reviewable diffs, granular attribute changes (type, length, precision/scale, unsigned, nullability, default, auto‑increment, collation).

## Related Issues