Get Model Catalog

Return the authoritative model catalog with an ETag for conditional GETs.

This is the source of truth the desktop backend fetches at startup and after every QUADRILLION_API_KEY change, so new models ship via a cloud redeploy without requiring a desktop release. Availability filtering by the user's own credentials happens on the desktop, with one exception: eap: true models are omitted server-side for accounts without the eap_access entitlement, because shipped desktop builds cannot be trusted to hide them. The ETag is therefore stable per entitlement variant rather than across all accounts.

Server-only data is stripped first: listed: false billing-only models and the inline pricing block never leave the server.

The ETag is derived from the stripped catalog content; an unchanged catalog returns 304 Not Modified so the desktop leaves its local store untouched.