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>

priv/repo/migrations/20260227212335_add_image_metadata.exs

@@ -0,0 +1,11 @@
+defmodule Berrypod.Repo.Migrations.AddImageMetadata do
+  use Ecto.Migration
+
+  def change do
+    alter table(:images) do
+      add :alt, :string
+      add :caption, :string
+      add :tags, :string
+    end
+  end
+end