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/mix/tasks/berrypod/backfill_alt_text.ex

@@ -0,0 +1,87 @@
+defmodule Mix.Tasks.Berrypod.BackfillAltText do
+  @shortdoc "Backfill alt text on existing images"
+  @moduledoc """
+  One-time task to populate alt text on images that were uploaded before
+  the alt field existed.
+
+  - Product images: copies alt from `product_images.alt`, falls back to product title
+  - Logo: uses site name from theme settings
+  - Header: "Header background"
+  - Icon: "Site icon"
+  - Skips images that already have alt text set
+
+  ## Usage
+
+      mix berrypod.backfill_alt_text
+  """
+
+  use Mix.Task
+
+  alias Berrypod.Repo
+  alias Berrypod.Media.Image
+  alias Berrypod.Products.ProductImage
+  alias Berrypod.Products.Product
+
+  import Ecto.Query
+
+  @impl true
+  def run(_args) do
+    Mix.Task.run("app.start")
+
+    backfill_product_images()
+    backfill_theme_images()
+
+    Mix.shell().info("Alt text backfill complete.")
+  end
+
+  defp backfill_product_images do
+    # Find product images with linked image records missing alt text
+    results =
+      from(pi in ProductImage,
+        join: i in Image,
+        on: i.id == pi.image_id,
+        join: p in Product,
+        on: p.id == pi.product_id,
+        where: is_nil(i.alt) and not is_nil(pi.image_id),
+        select: {i.id, pi.alt, p.title}
+      )
+      |> Repo.all()
+
+    count =
+      Enum.reduce(results, 0, fn {image_id, pi_alt, product_title}, acc ->
+        alt = pi_alt || product_title || "Product image"
+
+        from(i in Image, where: i.id == ^image_id)
+        |> Repo.update_all(set: [alt: alt])
+
+        acc + 1
+      end)
+
+    Mix.shell().info("  Updated #{count} product image(s)")
+  end
+
+  defp backfill_theme_images do
+    theme = Berrypod.Settings.get_theme_settings()
+
+    mapping = [
+      {theme.logo_image_id, theme.site_name || "Shop logo"},
+      {theme.header_image_id, "Header background"},
+      {theme.icon_image_id, "Site icon"}
+    ]
+
+    count =
+      Enum.reduce(mapping, 0, fn {image_id, alt}, acc ->
+        if image_id do
+          {updated, _} =
+            from(i in Image, where: i.id == ^image_id and is_nil(i.alt))
+            |> Repo.update_all(set: [alt: alt])
+
+          acc + updated
+        else
+          acc
+        end
+      end)
+
+    Mix.shell().info("  Updated #{count} theme image(s)")
+  end
+end