Get Onboard Sources

Return per-source warm history for a specific org the caller belongs to.

The dashboard pairs this with its hardcoded source catalog to show 'Last warmed: 2h ago, +127 templates' next to each integration tile.

Prefer ?org_id=<numeric>. ?slug=<org-slug> is accepted as a one-release deprecation path (resolved against the caller's memberships, 409 slug_ambiguous on collision). Without either, we return an empty list rather than guessing — guessing leaks data when a user belongs to multiple orgs.