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/lib/i18n/locales/en.ts

@@ -962,6 +962,10 @@ export default {
       artist_name: "Artist name",
       avatar: "Avatar",
       banner: "Banner",
+      model_photo: "Model photo",
+      model_photo_hint: "Used in model cards and on the model profile page. Avatar is used for comments and article authors.",
+      model_photo_uploaded: "Model photo uploaded",
+      model_photo_failed: "Model photo upload failed",
       is_admin: "Administrator",
       is_admin_hint: "Grants full admin access to the dashboard",
       photos: "Photo gallery",