feat: add shadow AI benchmarking for admin model comparison

Add a new admin-only feature that allows the platform owner to benchmark
the production AI model against up to 2 alternate models (any OpenAI-compatible
API) using real tenant data, without impacting users.

Backend:
- Shared AI caller utility (ai-caller.ts) for OpenAI-compatible endpoints
- Shadow AI module with service, controller, and 3 entities
- 6 admin API endpoints for model config CRUD, run trigger, and history
- Auto-creates shadow_ai_models, shadow_runs, shadow_run_results tables
- Exposes health-scores and investment-planning prompt builders for reuse

Frontend:
- New admin page at /admin/shadow-ai with 3 tabs:
  - Model Configuration (production + 2 alternate slots)
  - Run Comparison (tenant select, feature select, side-by-side results)
  - History (filterable run log with detail drill-down)
- Full side-by-side output display with diff highlighting
- Sidebar navigation link for AI Benchmarking

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

backend/src/modules/shadow-ai/shadow-ai.module.ts

@@ -0,0 +1,26 @@
+import { Module, OnModuleInit } from '@nestjs/common';
+import { TypeOrmModule } from '@nestjs/typeorm';
+import { ShadowAiController } from './shadow-ai.controller';
+import { ShadowAiService } from './shadow-ai.service';
+import { ShadowAiModel } from './entities/shadow-ai-model.entity';
+import { ShadowRun } from './entities/shadow-run.entity';
+import { ShadowRunResult } from './entities/shadow-run-result.entity';
+import { HealthScoresModule } from '../health-scores/health-scores.module';
+import { UsersModule } from '../users/users.module';
+
+@Module({
+  imports: [
+    TypeOrmModule.forFeature([ShadowAiModel, ShadowRun, ShadowRunResult]),
+    HealthScoresModule,
+    UsersModule,
+  ],
+  controllers: [ShadowAiController],
+  providers: [ShadowAiService],
+})
+export class ShadowAiModule implements OnModuleInit {
+  constructor(private shadowAiService: ShadowAiService) {}
+
+  async onModuleInit() {
+    await this.shadowAiService.ensureTables();
+  }
+}