Admin logs list no longer joins prompt/completion content for every row —
content is fetched per-row only when a log is expanded in the dashboard,
since a table of chat-length blobs was slow to serialize even at low row
counts.
request_logs.user_id (previously unpopulated despite an index existing on
it) is now set from the owning key on every insert, including synthetic
dashboard keys, and log queries filter on it directly instead of a
virtual_keys subquery. Pagination switches from OFFSET to a (created_at, id)
keyset cursor so paging deep into the log stays cheap. Adds indexes on
provider_name/key_prefix used by the admin filters.
