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.

1 files changed, 26 insertions, 9 deletions

diff --git a/public/stylesheets/admin.css b/public/stylesheets/admin.css
index 0e11747..b171800 100644
--- a/public/stylesheets/admin.css
+++ b/public/stylesheets/admin.css
@@ -1327,15 +1327,6 @@ div#image_browser ul li {
   height: 1rem;
 }
 
-.related_asset_headline_badge {
-  background: #fff3cd;
-  color: #6b5900;
-  border-radius: 999px;
-  padding: 0.1rem 0.6rem;
-  font-size: 0.75em;
-  white-space: nowrap;
-}
-
 .related_asset_remove {
   display: flex;
   align-items: center;
@@ -1372,6 +1363,32 @@ div#image_browser ul li {
   border-radius: 4px;
 }
 
+.related_asset_set_headline {
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  background: none;
+  border: none;
+  padding: 2px;
+  cursor: pointer;
+  color: #969696;
+} 
+
+.related_asset_set_headline svg {
+  width: 0.9rem;
+  height: 0.9rem;
+}
+
+.related_asset_set_headline:hover,
+.related_asset_set_headline[aria-pressed="true"] {
+  color: #6b5900;
+}
+
+.thumbnail_list li.is_headline {
+  border-color: #fff3cd;
+  background-color: #fffdf5;
+}
+
 /* ============================================================
    Live edit preview
    ============================================================ */