[FIX] om_account_followup: defer translation for email_subject default

[innovara]

The email_subject field is defined with default=_('Invoices Reminder'), which evaluates the translation at module load time. This triggers warnings about missing translation language and prevents proper translation at runtime.

Using default=lambda self: _('Invoices Reminder') defers evaluation until record creation, ensuring correct translation in the user’s language and removing the warning.

For reference, the warning looks like this:

WARNING innovara odoo.tools.translate: no translation language detected, skipping translation <frame at 0x7758ce5943c0, file '.../custom-addons/odoomates/om_account_followup/wizard/followup_print.py', line 34, code FollowupPrint> 
[...]
File ".../custom-addons/odoomates/om_account_followup/wizard/followup_print.py", line 34, in FollowupPrint
 default=_('Invoices Reminder'))
File ".../odoo/odoo/tools/translate.py", line 621, in get_text_alias
  module, lang = _get_translation_source(1)
File ".../odoo/tools/translate.py", line 610, in _get_translation_source
  lang = lang or _get_lang(frame, default_lang)
File ".../odoo/tools/translate.py", line 601, in _get_lang
  _logger.log(log_level, 'no translation language detected, skipping translation %s', frame, stack_info=True)