Add UK API endpoints

[MaxGhenis]

Summary

Add UK-specific API endpoints to the FastAPI backend.

Implementation

Update api/main.py and api/schemas.py with:

New Endpoints

• POST /api/uk/calculate - Calculate UK tax impact of Gift Aid donations
• POST /api/uk/target-donation - Calculate required donation for target net income
• GET /api/uk/regions - Get list of UK regions (England, Scotland, Wales, NI)
• GET /api/uk/tax-programs - Get UK Gift Aid program information

New Schemas

• UKIncomeInput - UK income sources (employment_income, self_employment_income, etc.)
• UKCalculateRequest - Request body for UK calculations
• UKCalculateResponse - Response with Gift Aid metrics

Key Differences from US Endpoints

• region instead of state_code
• gift_aid instead of charitable_cash_donations
• Include charity Gift Aid reclaim amount in response
• Include donor tax relief amount in response

Acceptance Criteria

• TDD: Tests written first
• /api/uk/calculate returns correct tax savings
• /api/uk/regions returns all 4 UK nations
• Response includes both charity reclaim and donor relief
• Proper error handling for invalid inputs

Part of #59