[bug] Sync fails for zero-token error entries

[seunggabi]

Problem

During bulk sync, a portion of entries consistently fail:

✔ Found 18,249 usage entries
⚠ Synced 12,255 entries (5994 failed)

Root Cause

The usage scanner includes entries with zero total tokens (model <synthetic>, API error responses like billing errors). These entries have no meaningful usage data and are rejected by the server during bulk import.

Solution

Filter out entries where total tokens (input + output + cache) is 0 in validateUsageData().

Verification

After fix, all entries sync successfully:

✔ Found 17,902 usage entries
✔ Successfully imported 17,902 entries