Add segment breakdown to tokens app dashboard
- Add by_segment aggregation in routes.py to group token usage by segment key
- Add Avg/Segment metric to summary card (shows average cost per attributed segment)
- Add By Segment table with filter, showing segment, requests, tokens, cost, and models
- Format segment keys as "HH:MM:SS (Xm)" for readability with raw key in tooltip
- Group entries without segment field under "[unattributed]" with gray styling
- Clean up unused imports (timedelta, List)
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>