add admin media library with image management and block picker integration

- Schema: alt, caption, tags fields on images table with metadata changeset
- Context: list_images with filters, find_usages, used_image_ids, delete_with_cleanup
- Admin UI: /admin/media with grid view, upload, filters, detail panel, metadata editing
- Block editor: :image field type for image_text and content_body blocks
- Page renderer: image_id resolution with legacy URL fallback
- Mobile: bottom sheet detail panel with slide-up animation
- CSS: uses correct --t-* admin theme tokens, admin-badge colour variants

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

lib/berrypod_web/components/layouts/admin.html.heex

@@ -102,6 +102,14 @@
               <.icon name="hero-document" class="size-5" /> Pages
             </.link>
           </li>
+          <li>
+            <.link
+              navigate={~p"/admin/media"}
+              class={admin_nav_active?(@current_path, "/admin/media")}
+            >
+              <.icon name="hero-photo" class="size-5" /> Media
+            </.link>
+          </li>
           <li>
             <.link
               href={~p"/admin/theme"}