Manuscript.date max_length=50 causes import failure for some manuscripts

When running `import_data manuscripts` locally, the import fails for manuscripts
whose `date` field in CantusDB exceeds 50 characters. The `Manuscript` model
currently defines:

date = models.CharField(max_length=50, blank=True, null=True)

This silently truncates or raises a database error depending on the backend
configuration.

**Steps to reproduce:**
1. Follow the local setup instructions in the README
2. Run `docker compose exec app python manage.py import_data manuscripts`
3. Observe failure for manuscripts with long date strings

**Expected behaviour:** Import completes for all manuscripts, or skips
problematic records gracefully with a warning.

**Suggested fix:** Increase `max_length` to 255 (or remove the limit
entirely with `TextField`) and generate the corresponding migration.