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/backend/src/graphql/resolvers/users.ts

@@ -134,6 +134,7 @@ builder.mutationField("adminUpdateUser", (t) =>
       artistName: t.arg.string(),
       avatarId: t.arg.string(),
       bannerId: t.arg.string(),
+      photoId: t.arg.string(),
     },
     resolve: async (_root, args, ctx) => {
       requireAdmin(ctx);
@@ -149,6 +150,7 @@ builder.mutationField("adminUpdateUser", (t) =>
         updates.artist_name = args.artistName;
       if (args.avatarId !== undefined && args.avatarId !== null) updates.avatar = args.avatarId;
       if (args.bannerId !== undefined && args.bannerId !== null) updates.banner = args.bannerId;
+      if (args.photoId !== undefined && args.photoId !== null) updates.photo = args.photoId;
 
       const updated = await ctx.db
         .update(users)