feat(usage): attribute Sources rows to user accounts in admin view (#9935)

The merged feature (#9920) let admins see per-API-key and per-source
totals but did not surface which user owned each key, and lumped
every user's Web UI traffic into a single global Web UI row. This
makes the admin Sources tab properly per-user attributable:

- KeyTotal gains UserID + UserName, populated from the snapshot the
  usage middleware already records. The by_key roll-up now groups by
  (api_key_id, api_key_name, user_id, user_name).
- New SourceTotals.ByUserSource roll-up groups (source, user_id,
  user_name) for sources without a key identity (web, legacy). Only
  populated on the admin path (includeLegacy=true); the non-admin
  endpoint stays unchanged for backwards compatibility.
- SourcesTable accepts showUserColumn={isAdmin}; admin view renders
  a User column, makes the search match user name/id, and expands
  Web UI / legacy pseudo-rows from the global aggregate to one row
  per user using by_user_source.

Refs: #9862

Signed-off-by: Ettore Di Giacinto <mudler@localai.io>
Co-authored-by: Ettore Di Giacinto <mudler@localai.io>