feat: async AI calls, 10-min timeout, and failure messaging

- Make all AI endpoints (health scores + investment recommendations)
  fire-and-forget: POST returns immediately, frontend polls for results
- Extend AI API timeout from 2-5 min to 10 min for both services
- Add "last analysis failed — showing cached data" message to the
  Investment Recommendations panel (matches health score widgets)
- Add status/error_message columns to ai_recommendations table
- Remove nginx AI timeout overrides (no longer needed)
- Users can now navigate away during AI processing without interruption

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

backend/src/database/tenant-schema.service.ts

@@ -325,6 +325,8 @@ export class TenantSchemaService {
         risk_notes JSONB,
         requested_by UUID,
         response_time_ms INTEGER,
+        status VARCHAR(20) DEFAULT 'complete',
+        error_message TEXT,
         created_at TIMESTAMPTZ DEFAULT NOW()
       )`,