feat : added email subjects (M2-9301, M2-9726, M2-9731)

[sricharan-varanasi]

• Tests for the changes have been added
• Related documentation has been added / updated
• For new features, QA automation engineers have been tagged
• OSS packages added to
  Curious open source credit page

📝 Description

🔗 Jira Ticket M2-9301
🔗 Jira Ticket M2-9726
🔗 Jira Ticket M2-9731

This PR removes hardcoded email subjects for reset password and assignment notifications, invitations and replaces them with localized text templates. This aligns subject handling with how email bodies are already localized, making it easier to add new languages without code changes.
Changes include:

• Added new subject templates under src/apps/mailing/static/subjects/:
  • reset_password_en.txt, reset_password_fr.txt, reset_password_el.txt, reset_password_es.txt, reset_password_pt.txt
  • assignment_notification_en.txt, assignment_notification_fr.txt, assignment_notification_el.txt, assignment_notification_es.txt, assignment_notification_pt.txt
  • invitation_en.txt, inviatation_fr.txt, invitation_el.txt, invitation_es.txt, invitation_pt.txt
• Updated MailingService to support loading localized .txt subject templates with fallback to English.
• Updated PasswordRecoveryService.send_password_recovery to use the new template-based subject instead of hardcoded dict.
• Updated ActivityAssignmentService.send_email_notification to use the new template-based subject instead of _get_email_assignment_subject.
• Removed now-unused _get_email_assignment_subject and get_locallized_email_subject functions.

🪤 Peer Testing

• Trigger reset password, assignment notification, and invitation emails for accounts in each supported language (en, fr, el, es, pt).
• Verify the subject line comes from the correct .txt template and matches the localized body.
• Check that missing languages gracefully fall back to English.
• Confirm all email types (reset, assignment, invitation) send successfully.

✏️ Notes

• New .txt files must be packaged for deployment

[aweiland]

👍 Looks good

[maxto024]

Looks good

[github-actions]

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