feat(insights): add cost breakdown by role

[subomi]

Summary

• Adds a group_by=role parameter to the searchUsers telemetry endpoint that aggregates per-user costs by RBAC role
• Backend fetches per-user costs from ClickHouse, joins with role assignments from Postgres, and returns role-level aggregates (cost, tokens, sessions, user count)
• Frontend gains an Employee | Role toggle on the "Usage by Employee" table that lazy-loads role data when activated

Test plan

• Verify Employee view (default) works unchanged
• Toggle to Role view — confirm roles appear with correct cost/token aggregates
• Verify "Unassigned" bucket appears for users without role assignments
• Confirm role names resolve correctly from the existing roles API
• Test sorting on all role columns (Role, Users, Total Cost, Cost/User, Input Tokens, Output Tokens, Sessions)
• Verify pagination works in role view
• Confirm switching between Employee/Role resets sort and page state

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