fix : Revenue Breakdown: Deduct Business Expenses for Correct Tax Estimate and Safe-to-Spend #339

[jenilbanavani]

Fixes #339

Summary

Implements #339 by deducting recurring business expenses before estimating income tax.

Changes

• Added business_expenses and taxable_profit to SpendableIncome.
• Updated compute_spendable_income() to deduct recurring expenses before computing income tax.
• Updated the Tax intent to pass recurring expenses into the spendable income calculation.
• Updated the Tax & Reserves waterfall to display Business Expenses and Taxable Profit.
  *Added tests covering recurring expense deduction, monthly/yearly normalization, and empty expense lists..

Manual verification

• Verified the Tax & Reserves waterfall using a recurring expense.
• Confirmed Business Expenses reduce Taxable Profit before income tax is estimated.
• Confirmed Safe to Spend reflects the corrected calculation.

AI Assistance

I used an AI coding assistant to help explore the codebase and draft parts of the implementation. I reviewed the implementation, tested the changes, and understand all submitted code.

Checklist

• [X 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>").
• I understand and can explain all submitted changes; if AI assistance was significant, I have disclosed it in the summary above.