Multi-machine submissions overwrite daily data instead of merging

[kenshin579]

Problem

When a user uploads cc.json from multiple machines under the same GitHub username, daily data for overlapping dates is overwritten instead of being summed/appended.

Current behavior

The merge logic in convex/submissions.ts uses Map.set() to handle overlapping dates:

ccData.daily.forEach(day => {
  existingDailyMap.set(day.date, { ... }); // overwrites existing data
});

This means:

• Same machine, account switch: Works fine — ccusage reads all local logs, so cc.json already contains combined data
• Different machines, non-overlapping dates: Creates a separate submission — data is preserved but split
• Different machines, overlapping dates: Second upload replaces the first machine's data for shared dates → data loss

Example

1. Machine A: used Claude on 2025-03-01 ($20), 2025-03-02 ($15)
2. Machine B: used Claude on 2025-03-02 ($10), 2025-03-03 ($25)
3. Upload Machine A's cc.json → stored correctly
4. Upload Machine B's cc.json → 2025-03-02 becomes $10 (Machine A's $15 is lost)

Expected: 2025-03-02 should be $25 ($15 + $10)

Considerations

A simple "sum on overlap" approach introduces a new problem: re-uploading from the same machine would double-count data. Possible solutions:

1. Track submissions per machine — add a machine identifier so the system knows which data to replace vs. append
2. Store separate submissions per source/machine — keep them independent and aggregate at query time
3. Deduplicate by session/conversation ID — if ccusage provides session-level granularity

Environment

• Scenario: User with multiple machines (e.g., work desktop + personal laptop) under the same GitHub account
• Submission method: Both CLI and OAuth affected