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/art/upload.html.twig

@@ -0,0 +1,23 @@
+{% extends 'base.html.twig' %}
+
+{% block title %}Upload art · dmtools{% endblock %}
+
+{% block body %}
+    <div class="mb-3">
+        <a href="{{ path('app_art') }}" class="text-decoration-none small">&larr; Art</a>
+        <h1 class="h3 mb-0">Upload art</h1>
+    </div>
+
+    <div class="card shadow-sm" style="max-width: 32rem;">
+        <div class="card-body">
+            {{ form_start(form, {attr: {enctype: 'multipart/form-data'}}) }}
+            {{ form_row(form.name) }}
+            {{ form_row(form.file) }}
+            <div class="mt-3 d-flex gap-2">
+                <button class="btn btn-primary">Upload</button>
+                <a href="{{ path('app_art') }}" class="btn btn-outline-secondary">Cancel</a>
+            </div>
+            {{ form_end(form) }}
+        </div>
+    </div>
+{% endblock %}