docs: document the generation model catalogue endpoints

[dazzatronus]

The model catalogue endpoints ship in the API but appear in no published schema, so the clients meant to drop their hardcoded model lists have nothing to generate against.

Documented as they behave, read off the merged handler rather than the design.

Two filters the handler accepts are left undocumented and a follow-up removes them. status cannot vary: no model carries one, and the handler reads it through a cast to a property the interface never declares, so everything resolves to active. type works, but each entry already carries its type and the catalogue is a dozen cached entries, so the filter saves a client nothing.

expand stays — option schemas are the bulk of an entry.

Pricing is its own schema because its shape carries a rule: credits is a number when a model charges flatly and an object keyed by option value when it does not, with tieredBy naming the selector. That tells a reader there is no formula to evaluate.

options stays free-form: it holds a per-model schema the registry generates, and pinning it here would duplicate a contract that can only drift.

Verify: pnpm build && pnpm test && pnpm test:smoke — 71 checks pass, both operations reach the generated types.