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>

package.json

@@ -1,6 +1,6 @@
 {
   "name": "epicure",
-  "version": "0.73.0",
+  "version": "0.73.1",
   "private": true,
   "scripts": {
     "dev": "pnpm --filter web dev",