fix: use invoicing role contact for invoice email

[kocaemre]

Summary

Fixes #487.

The invoice email path only looked at the legacy client.invoicing_contact relationship. The current contacts UI can also attach contacts through the ClientContact association with role invoicing, which meant a client could visibly have an invoicing contact with an email address while Send Invoice still failed with “No contact email available”.

This adds a shared invoice-recipient contact lookup that:

• keeps preferring the legacy direct invoicing contact
• falls back to a many-to-many ClientContact whose role is invoicing
• reuses that lookup for invoice/reminder email composition and invoice recipient name/address helpers
• adds regression coverage for generating invoice emails from an invoicing-role contact

AI assistance was used while drafting this change; I reviewed the code path and verified the final patch locally.

Checklist

• I have read the Contributing guide.
• I have installed and tested the application for my platform (just dev).
  • Not run: this is a backend/model email-recipient fix and just is not installed in this environment.
• The test suite passes locally (just test).
  • Equivalent run: pytest -q → 549 passed, 1 skipped.
• Pre-commit hooks are installed and pass (just precommit).
  • Not run: just is not installed in this environment. Ran ruff check ... and git diff --check instead.
• 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>").
  • Not applicable: this only adds Python properties/helper logic; no SQLModel fields, tables, or relationships changed.
• If my change affects the graphical user interface, I have provided screenshots.
  • Not applicable: no UI files changed.
• I understand and can explain all submitted changes; if AI assistance was significant, I have disclosed it in the summary above.

Verification

uv sync --dev
source .venv/bin/activate
python -m py_compile tuttle/model.py tuttle/invoicing.py tuttle/app/invoicing/intent.py tuttle_tests/test_invoice.py
pytest tuttle_tests/test_invoice.py -q
pytest -q
ruff check tuttle/model.py tuttle/invoicing.py tuttle/app/invoicing/intent.py tuttle_tests/test_invoice.py
git diff --check

Results:

• pytest tuttle_tests/test_invoice.py -q: 41 passed
• pytest -q: 549 passed, 1 skipped
• ruff check ...: All checks passed
• git diff --check: no whitespace errors