Every listing the workspace has approved for that portal, in the portal's own
format. The URL is the credential: the token is issued per portal account and can
be rotated in the CRM, which retires the old URL immediately.

Two response headers report what the feed contains, so an agency curling it can see
at once that listings were withheld and go and fix them:

• x-listing-count — listings included.
• x-skipped-count — listings left out because they failed the portal's own
  validation (a missing permit number, no photos, an unsupported rent frequency).

Rate limited to 120 requests a minute per provider and caller.