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/modules/board-planning/board-planning.module.ts

@@ -0,0 +1,11 @@
+import { Module } from '@nestjs/common';
+import { BoardPlanningController } from './board-planning.controller';
+import { BoardPlanningService } from './board-planning.service';
+import { BoardPlanningProjectionService } from './board-planning-projection.service';
+
+@Module({
+  controllers: [BoardPlanningController],
+  providers: [BoardPlanningService, BoardPlanningProjectionService],
+  exports: [BoardPlanningService],
+})
+export class BoardPlanningModule {}