feat(newsletter): fix immediate-send being counted in newsletter rate…

[BhaveshKukreja29]

Closes #149 #142 #141

• mxgo/crud.py: added count_recurring_tasks_for_user to distinguish recurring from one-time tasks.
• mxgo/schemas.py: added language field to newsletter request.
• mxgo/api.py: refactored immediate sends to skip scheduling, implemented LLM subject generation, and added date context to research prompts.
• tests/test_api.py: updated mocks and assertions to match new task-counting logic and verification methods.

Checklist

• I have created an issue for this change (not mandatory for small changes)
• My changes are to-the-point
• Code is styled as the rest of the codebase and linting passes
• I have tested my changes locally and they work as expected
• I have updated the documentation (if needed)
• I have reviewed the code myself once and I don't see any

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