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>

backend/src/app.module.ts

@@ -28,6 +28,7 @@ import { MonthlyActualsModule } from './modules/monthly-actuals/monthly-actuals.
 import { AttachmentsModule } from './modules/attachments/attachments.module';
 import { InvestmentPlanningModule } from './modules/investment-planning/investment-planning.module';
 import { HealthScoresModule } from './modules/health-scores/health-scores.module';
+import { BoardPlanningModule } from './modules/board-planning/board-planning.module';
 import { ScheduleModule } from '@nestjs/schedule';
 
 @Module({
@@ -79,6 +80,7 @@ import { ScheduleModule } from '@nestjs/schedule';
     AttachmentsModule,
     InvestmentPlanningModule,
     HealthScoresModule,
+    BoardPlanningModule,
     ScheduleModule.forRoot(),
   ],
   controllers: [AppController],