Sources semantically similar to an anchor source

Pulls the anchor source's vector from the polymorphic ENTITIES_INDEX (orgs + products + sources) via getByIds, then runs query restricted to type=source so the candidate pool isn't wasted on parent orgs/products that would be filtered out post-hydration anyway. scope=org adds an org_id filter; scope=global (default) keeps the source filter only.

Degrades gracefully the same way /v1/related/releases does — missing binding, unembedded anchor, or query error returns { degraded: true, degradedReason, items: [] } with HTTP 200.

Each surviving row carries its source basics plus an org rollup and a recent-release window-function stat block (releaseCount, latestDate, latestTitle, latestVersion, recentCount) so a rail card can render without follow-up requests.