feat(platform-service): shared onboarding analytics module (Phase 4.3) — 8 tests
New modules/onboarding/ with standard pattern (types → repository → routes):
- POST /onboarding/step — track step completion (any auth)
- POST /onboarding/complete — track onboarding completion (any auth)
- GET /onboarding/funnel — funnel conversion rates (admin)
- GET /onboarding/user/:userId — user progress (admin)
- Cosmos containers: onboarding_events, onboarding_completions
- Funnel: per-step unique users, conversion rates, avg duration
- 8 tests: schema validation, funnel, date filter, user progress, empty product
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