feat: add Future Year Budget Planning with inflation-adjusted projections

Adds budget planning capability under Board Planning, allowing HOA boards
to model future year budgets with configurable per-year inflation rates.

Backend:
- New budget_plans + budget_plan_lines tables (migration 014)
- BudgetPlanningService: CRUD, inflation generation (per-month preservation),
  status workflow (planning → approved → ratified), ratify-to-official copy
- 8 new API endpoints on board-planning controller
- Projection engine (both board-planning and reports) now falls back to
  planned budgets via UNION ALL query when no official budget exists
- Extended year range from 3 to dynamic based on projection months

Frontend:
- BudgetPlanningPage with monthly grid table (mirrors BudgetsPage pattern)
- Year selector, inflation rate control, status progression buttons
- Inline editing with save, confirmation modals for status changes
- Manual edit tracking with visual indicator
- Summary cards for income/expense totals

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/components/layout/Sidebar.tsx

@@ -66,6 +66,7 @@ const navSections = [
   {
     label: 'Board Planning',
     items: [
+      { label: 'Budget Planning', icon: IconReportAnalytics, path: '/board-planning/budgets' },
       { label: 'Investment Scenarios', icon: IconScale, path: '/board-planning/investments' },
       { label: 'Assessment Scenarios', icon: IconCalculator, path: '/board-planning/assessments' },
       { label: 'Compare Scenarios', icon: IconGitCompare, path: '/board-planning/compare' },