Update EmailTemplate mocking in all test files

- Replace @patch("..tasks.EmailTemplate") with sent_emails fixture
- Add EmailTemplateFactory instances with correct identifiers
- Replace mock assertions with sent_emails() assertions
- Verify template identifier, conference, recipients, and placeholders

This ensures tests use real EmailTemplate instances instead of mocks,
improving test reliability and coverage.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Marco Acierno <[email protected]>

Author: claude[bot]