feat: add dedicated model photo separate from avatar

Adds a `photo` field to the users table (and a migration) that serves
as a dedicated profile/card image for models. This is now used in model
cards and on the model single page, while `avatar` is reserved for
comments, article authors, and the user profile page.

- DB: `photo` column on `users` with FK to `files`
- GraphQL: exposed on ModelType, UserType, AdminUserDetailType; photoId arg on adminUpdateUser
- Services: photo field in MODELS_QUERY, MODEL_BY_SLUG_QUERY, ADMIN_GET/UPDATE_USER
- Frontend: model cards and single page use `photo ?? avatar` fallback
- Admin: model photo upload section in user edit page

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

packages/frontend/src/routes/models/+page.svelte

@@ -110,7 +110,7 @@
           >
             <div class="relative">
               <img
-                src={getAssetUrl(model.avatar, "preview")}
+                src={getAssetUrl(model.photo ?? model.avatar, "preview")}
                 alt={model.artist_name}
                 class="w-full aspect-square object-cover group-hover:scale-105 transition-transform duration-300 bg-muted"
               />