The get_token_usage_by_user query lacked group_id filtering, while the
companion get_message_count_by_user query already supported it. When an
admin filtered analytics by user group, message counts were correctly
scoped to the group but token usage totals included data from all users.
Add the group_id parameter and subquery filter to get_token_usage_by_user,
matching the pattern used by get_message_count_by_user and other analytics
queries, and pass group_id through from the analytics endpoint.
- Add chat_message table for message-level analytics with usage JSON field
- Add migration to backfill from existing chats
- Add /analytics endpoints: summary, models, users, daily
- Support hourly/daily granularity for time-series data
- Fill missing days/hours in date range
