Record invoice sent date in timeline

[sreedevi-i]

Summary

Fixes #393
Adds a recorded sent_date for invoices, updates sent/un-sent behavior, shows the sent date in invoice details, and adds invoice sent events to the timeline.

Approach

This adds a nullable sent_date field to invoices so Tuttle can record when an invoice was sent, instead of only storing a boolean sent status.

When an invoice is marked sent or sent by email, sent_date is set to today. When it is unmarked as sent, sent_date is cleared.

The timeline now emits an Invoice ... sent event for regular invoices using sent_date, falling back to the invoice date for older sent invoices that do not have a recorded sent date.

Invoice details also show the sent date when present.

Since this change modifies the schema in tuttle/model.py (adding sent_date to Invoice), I generated and reviewed the resulting migration (revision 4447db01f757): it's a straightforward nullable add_column on invoice, no rename risk, and PRAGMA foreign_key_check passes.

Developed with AI assistance (Cursor). I've reviewed and can explain all changes.

Screenshots

Invoice details showing recorded sent date:

Timeline showing invoice sent event:

Checklist

• I have read the Contributing guide.
• I have installed and tested the application for my platform (just dev).
• The test suite passes locally (just test).
• Pre-commit hooks are installed and pass (just precommit).
• I have added or updated tests where appropriate.
• I have updated the documentation / docstrings where appropriate.
• If my change touches the schema (tuttle/model.py), I generated and reviewed an Alembic migration (just migrate "<msg>").
• If my change affects the graphical user interface, I have provided screenshots.
• I understand and can explain all submitted changes; if AI assistance was significant, I have disclosed it in the summary above.

[clstaudt]

Thanks @sreedevi-i - good work, clean PR, good to go.

[sreedevi-i]

Thanks for the review!