Make headline images explicit, add asset credits

- related_assets gains a `headline` boolean (DB-enforced: at most one
  per page), replacing "first image by position" as the headline
  rule. A rake task backfills the current first image on every live
  head/draft, so nothing changes visually until an editor changes it.
- The image picker sidebar gets a star toggle reflecting the flag;
  the TinyMCE inline-image picker's badge now reads it too, instead
  of assuming position 0.
- No headline chosen (or none attached) now falls back to the
  gallery-count caption itself becoming the lightbox trigger, instead
  of the gallery being unreachable.
- Assets gain creator, source_url, and license_key (against a new
  config/asset_licenses.yml dictionary). asset_credit renders a
  degrading attribution line, reused as a hidden per-image glightbox
  caption so credit is one click away for every image, not only the
  headline's always-visible one.
- Fixed: asset thumbnails rendered unconditionally regardless of
  whether a real variant exists on disk. Asset#has_variant? checks
  file existence, not content type -- some legacy PDFs have real
  pre-rewrite thumbnails a content-type check would have hidden.
- assets#new/edit rebuilt onto the same node_description/node_content
  layout as assets#show, picking up the three new fields in the
  process.

2 files changed, 14 insertions, 0 deletions

diff --git a/db/migrate/20260720115941_add_credit_fields_to_assets.rb b/db/migrate/20260720115941_add_credit_fields_to_assets.rb
new file mode 100644
index 0000000..638d894
--- /dev/null
+++ b/db/migrate/20260720115941_add_credit_fields_to_assets.rb
@@ -0,0 +1,7 @@
+class AddCreditFieldsToAssets < ActiveRecord::Migration[8.1]
+  def change
+    add_column :assets, :license_key, :string
+    add_column :assets, :creator, :string
+    add_column :assets, :source_url, :string
+  end
+end
diff --git a/db/migrate/20260720120917_add_headline_to_related_assets.rb b/db/migrate/20260720120917_add_headline_to_related_assets.rb
new file mode 100644
index 0000000..c77e1ef
--- /dev/null
+++ b/db/migrate/20260720120917_add_headline_to_related_assets.rb
@@ -0,0 +1,7 @@
+class AddHeadlineToRelatedAssets < ActiveRecord::Migration[8.1]
+  def change
+    add_column :related_assets, :headline, :boolean, null: false, default: false
+    add_index :related_assets, :page_id, unique: true, where: "headline = true",
+               name: "index_related_assets_on_page_id_headline_uniqueness"
+  end
+end