Flutter SDK syncs 0 samples across 40 types — /sync endpoint never called, /logs returns 400

[csryan418]

Environment:
SDK version: 0.0.15
iOS: 26.5
iPhone 17 Pro
Backend: Open Wearables self-hosted (Docker), latest main

What happened:
After connecting successfully via invitation code and granting all HealthKit permissions, starting background sync completes immediately with "Sync complete: 0 samples across 40 types". No data is ever stored in the backend.
Checking backend logs reveals the SDK never calls the /api/v1/sdk/users/{user_id}/sync endpoint at all. The only SDK request hitting the backend is /api/v1/sdk/users/{user_id}/logs, which returns 400.

Backend logs: POST /api/v1/sdk/users/{user_id}/logs HTTP/1.1" 400
The /sync endpoint is never called.

Steps to reproduce:

1. Self-host Open Wearables backend via Docker
2. Create a user and generate an invitation code
3. Connect Flutter example app using host + invitation code
4. Grant all HealthKit permissions
5. Tap Start Sync

Expected: Health data from HealthKit synced to backend
Actual: Sync completes instantly with 0 samples, sync endpoint never called, logs endpoint returns 400