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/App.tsx

@@ -31,6 +31,11 @@ import { AssessmentGroupsPage } from './pages/assessment-groups/AssessmentGroups
 import { CashFlowForecastPage } from './pages/cash-flow/CashFlowForecastPage';
 import { MonthlyActualsPage } from './pages/monthly-actuals/MonthlyActualsPage';
 import { InvestmentPlanningPage } from './pages/investment-planning/InvestmentPlanningPage';
+import { InvestmentScenariosPage } from './pages/board-planning/InvestmentScenariosPage';
+import { InvestmentScenarioDetailPage } from './pages/board-planning/InvestmentScenarioDetailPage';
+import { AssessmentScenariosPage } from './pages/board-planning/AssessmentScenariosPage';
+import { AssessmentScenarioDetailPage } from './pages/board-planning/AssessmentScenarioDetailPage';
+import { ScenarioComparisonPage } from './pages/board-planning/ScenarioComparisonPage';
 
 function ProtectedRoute({ children }: { children: React.ReactNode }) {
   const token = useAuthStore((s) => s.token);
@@ -137,6 +142,11 @@ export function App() {
         <Route path="reports/sankey" element={<SankeyPage />} />
         <Route path="reports/year-end" element={<YearEndPage />} />
         <Route path="reports/quarterly" element={<QuarterlyReportPage />} />
+        <Route path="board-planning/investments" element={<InvestmentScenariosPage />} />
+        <Route path="board-planning/investments/:id" element={<InvestmentScenarioDetailPage />} />
+        <Route path="board-planning/assessments" element={<AssessmentScenariosPage />} />
+        <Route path="board-planning/assessments/:id" element={<AssessmentScenarioDetailPage />} />
+        <Route path="board-planning/compare" element={<ScenarioComparisonPage />} />
         <Route path="settings" element={<SettingsPage />} />
         <Route path="preferences" element={<UserPreferencesPage />} />
         <Route path="org-members" element={<OrgMembersPage />} />