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>

apps/web/lib/changelog.ts

@@ -1,5 +1,5 @@
 // Mirrors CHANGELOG.md at the repo root — update both together.
-export const APP_VERSION = "0.73.0";
+export const APP_VERSION = "0.73.1";
 
 export type ChangelogEntry = {
   version: string;
@@ -11,6 +11,13 @@ export type ChangelogEntry = {
 };
 
 export const CHANGELOG: ChangelogEntry[] = [
+  {
+    version: "0.73.1",
+    date: "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.",
+    ],
+  },
   {
     version: "0.73.0",
     date: "2026-07-23 14:00",