feat: add Board Planning module with investment/assessment scenario modeling

Implements Phase 11 Forecasting Tools - a "what-if" scenario planning system
for HOA boards to model financial decisions before committing.

Backend:
- 3 new tenant-scoped tables: board_scenarios, scenario_investments, scenario_assessments
- Migration script (013) for existing tenants
- Full CRUD service for scenarios, investments, and assessments
- Projection engine adapted from cash flow forecast with investment/assessment deltas
- Scenario comparison endpoint (up to 4 scenarios)
- Investment execution flow: converts planned → real investment_accounts + journal entry

Frontend:
- New "Board Planning" sidebar section with 3 pages
- Investment Scenarios: list, create, detail with investments table + timeline
- Assessment Scenarios: list, create, detail with changes table
- Scenario Comparison: multi-select overlay chart + summary metrics
- Shared components: ProjectionChart, InvestmentTimeline, ScenarioCard, forms
- AI Recommendation → Investment Plan integration (Story 1A)

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

frontend/src/components/layout/Sidebar.tsx

@@ -18,6 +18,9 @@ import {
   IconClipboardCheck,
   IconSparkles,
   IconHeartRateMonitor,
+  IconCalculator,
+  IconGitCompare,
+  IconScale,
 } from '@tabler/icons-react';
 import { useAuthStore } from '../../stores/authStore';
 
@@ -60,6 +63,14 @@ const navSections = [
       { label: 'Vendors', icon: IconUsers, path: '/vendors' },
     ],
   },
+  {
+    label: 'Board Planning',
+    items: [
+      { 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' },
+    ],
+  },
   {
     label: 'Reports',
     items: [