fix: gate Model Prefs picker behind BYOK access (v0.73.1)

Settings -> AI's model-selection form (per-use-case provider + model
ID picker) had no access gate -- every user could see and use it,
regardless of whether they had a BYOK key to route calls to or any
reason to care which model served a request. Now gated behind
isByokEnabled, same reasoning as BYOK itself: picking a specific
provider/model only makes sense once you have your own key. Hidden
entirely for everyone else, not locked-and-teased -- there's nothing
for a non-BYOK user to unlock here.

GET/PUT /api/v1/users/me/model-prefs switched from requireSession to
requireByok to match.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

CHANGELOG.md

@@ -2,6 +2,11 @@
 
 All notable changes to Epicure are documented here. This file is mirrored in-app at `/changelog` (and in the admin dashboard) via `apps/web/lib/changelog.ts` — update both together.
 
+## 0.73.1 — 2026-07-24 09:30
+
+### Fixed
+- Settings → AI's model picker (choose a specific provider/model like `gpt-4o-mini` or `o3-mini` per AI feature) had no access gate at all — every user could see and use it, regardless of whether they had any reason to. Now hidden unless you have BYOK access, since picking a specific model only matters once you've got your own key to route it to.
+
 ## 0.73.0 — 2026-07-23 14:00
 
 ### Added