fix(platform-service): missing cosmos containers + NaN guard in cross-product limit

- cosmos-init.ts: add onboarding_events + onboarding_completions containers
  (onboarding repository references them but they were never registered)
- telemetry cross-product route: parseInt(limit) could produce NaN when
  given non-numeric input — now clamps to 1-100 with default 20

services/platform-service/src/lib/cosmos-init.ts

@@ -147,6 +147,9 @@ const CONTAINER_DEFS: Record<string, ContainerConfig> = {
   diagnostic_insights: { partitionKeyPath: '/clusterId', defaultTtl: 90 * 86400 },
   diagnostic_queries: { partitionKeyPath: '/userId', defaultTtl: 30 * 86400 },
   proactive_alerts: { partitionKeyPath: '/productId', defaultTtl: 30 * 86400 },
+  // Onboarding analytics (Phase 4.3)
+  onboarding_events: { partitionKeyPath: '/productId' },
+  onboarding_completions: { partitionKeyPath: '/productId' },
   // Broadcast Messaging & Surveys (see docs/roadmaps/not-started/platform_BROADCAST_SURVEY_ROADMAP.md)
   broadcasts: { partitionKeyPath: '/productId' },
   broadcast_deliveries: { partitionKeyPath: '/userId', defaultTtl: 90 * 86400 },

services/platform-service/src/modules/telemetry/routes.ts

@@ -928,10 +928,14 @@ export async function telemetryRoutes(app: FastifyInstance) {
       throw new BadRequestError('Provide 1-20 product IDs');
     }
 
+    const parsedLimit = limit ? parseInt(limit, 10) : 20;
+    const clampedLimit =
+      Number.isFinite(parsedLimit) && parsedLimit > 0 ? Math.min(parsedLimit, 100) : 20;
+
     const [summary, daily, clusters] = await Promise.all([
       repo.queryCrossProductSummary(productIds, from, to),
       repo.queryCrossProductDaily(productIds, from, to),
-      repo.queryCrossProductClusters(productIds, limit ? parseInt(limit, 10) : 20),
+      repo.queryCrossProductClusters(productIds, clampedLimit),
     ]);
 
     return { summary, daily, clusters };