Add Art section: upload named images, gallery, delete

- Artwork entity (name, stored filename, original name, mime, size, date)
- /art gallery grid; /art/upload (name + image file, PNG/JPEG/GIF/WebP,
  20M cap); /art/{id}/file streams the image behind auth; /art/{id}/delete
  (CSRF) removes row + file
- files stored under var/uploads/art/ (git-ignored, on the php_var volume
  in Docker), served through a controller so they stay login-gated
- "Art" nav item

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

templates/base.html.twig

@@ -30,6 +30,7 @@
                             { route: 'app_characters', label: 'Characters' },
                             { route: 'app_npcs',      label: "NPC's" },
                             { route: 'app_enemies',   label: 'Enemies' },
+                            { route: 'app_art',       label: 'Art' },
                             { route: 'app_storyline', label: 'Storyline' },
                         ] %}
                         <ul class="navbar-nav me-auto">