Describe the bug

When a single user submits token usage from multiple machines (same GitHub account, different devices), the usage data is overwritten (for conflicting dates) instead of aggregated. This causes data loss for users who work across multiple machines.

Expected behavior

Usage from all machines should be summed for the same date: 1500 tokens

Idea Notes

• Per-device breakdown: Since each device already receives a unique API token, we could store usage per-device rather than merging into a single record. This provides better data integrity:
  • Each device owns its own data slice (no merge conflicts)
  • Submissions are idempotent per device
  • Totals are verifiable (sum of all devices)
  • Full audit trail of which device submitted what
• 💙 This issue was reported by @anaclumos and @Yeachan-Heo