feat(platform-service): cross-product telemetry endpoint (Phase 4.2) — 6 tests
GET /telemetry/cross-product?products=a,b,c — admin-only endpoint for
multi-product telemetry comparison:
- Per-product summary: totalEvents, errorEvents, errorRate, topModules, platforms
- Daily event counts per product (for stacked bar charts)
- Top error clusters across products (merged + sorted by count)
- Date range filtering via from/to query params
- 6 tests: summary, daily breakdown, clusters, date filter, unknown product, limit