feat(email): add HTML workspace invite email

[nancysangani]

Closes #442

📝 What does this PR do?

When a user is invited to a workspace, the backend now immediately dispatches a branded HTML invitation email with workspace details and a one-click acceptance link.

Changes:

• Added send_workspace_invite_email() to backend/app/email_service.py — builds a table-based HTML email with:
  • Indigo → purple branded header
  • Workspace name + invitation copy
  • Optional personal message block (only rendered when inviter adds a note)
  • Prominent "Accept Invitation →" CTA button
  • Amber expiry notice (e.g. "⏳ expires in 72 hours")
  • Auto-generated plain-text fallback for clients that don't render HTML
• Updated backend/app/routes/workspaces.py to call send_workspace_invite_email() instead of the old hand-rolled plain-text body

🗂️ Type of Change

• ✨ New feature
• 🔧 Refactor / code cleanup
• 🧪 Tests

🧪 How was this tested?

• Added / updated tests

9 unit tests added in backend/tests/test_workspace_invite_email.py:

Test	Asserts
test_delegates_to_send_email	send_email called exactly once
test_subject_contains_workspace_name	Subject includes workspace name
test_html_body_contains_invite_link	HTML contains acceptance URL
test_html_body_contains_workspace_name	HTML contains workspace name
test_html_body_contains_expiry	Expiry hours present in HTML
test_personal_message_included_in_html	Optional message block rendered
test_no_personal_message_omits_block	Block absent when no message
test_plain_text_body_contains_invite_link	Plain-text fallback has URL
test_recipient_passed_to_send_email	Correct recipient address

✅ Self-Review Checklist

• My branch is based on dev, not main
• I have not added any secrets / API keys
• I have not modified main branch or any HuggingFace deployment config
• My code follows the existing style (no unnecessary formatting changes)
• I have updated relevant docs / comments if needed

[nancysangani]

Hi @param20h, please review the PR when you get a chance. Thanks!

[github-actions]

🎉 Congratulations on getting your Pull Request merged! 🎉

Thank you for contributing to PDF-Assistant-RAG as part of GSSoC '26! 🚀

Keep up the great work! ✨