The upcoming /api/query RBAC patch (next phase) gates direct
`bq."<dataset>"."<source_table>"` references in user SQL — every such path
must point at a registered query_mode='remote' BigQuery row, otherwise the
caller has stepped around the registry and around RBAC.
Add `TableRegistryRepository.find_by_bq_path(bucket, source_table)` to
support that lookup. Returns None if no row matches, the row dict if
exactly one matches, or the oldest-by-`registered_at` row when 2+ match
(no UNIQUE constraint on `(source_type, bucket, source_table)` — admins
can in principle register a BQ table twice with different ids/names).
Match is case-insensitive on bucket+source_table so user SQL `SELECT FROM
bq.Finance.UE` resolves to a `(finance, ue)` registry row. NULL values in
either column are excluded so a legacy NULL-bucket row never masks a
legitimate non-NULL lookup.
5 RED tests cover: empty registry, non-BQ source rejected, single match,
oldest-of-many tie-breaker, case-insensitive match, NULL-column exclusion.
All initially failed with AttributeError; pass after the ~30 LOC method
addition.
