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/router.ex

@@ -232,6 +232,7 @@ defmodule BerrypodWeb.Router do
       live "/settings/email", Admin.EmailSettings, :index
       live "/pages", Admin.Pages.Index, :index
       live "/pages/:slug", Admin.Pages.Editor, :edit
+      live "/media", Admin.Media, :index
       live "/redirects", Admin.Redirects, :index
     end