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, 7 insertions, 1 deletions

diff --git a/app/models/asset.rb b/app/models/asset.rb
index aca0ee80..8bea1b3d 100644
--- a/app/models/asset.rb
+++ b/app/models/asset.rb
@@ -1,12 +1,18 @@
 class Asset < ApplicationRecord
+  IMAGE_CONTENT_TYPES = ["image/gif", "image/jpeg", "image/png", "image/webp"]
 
   include FileAttachment
   
   has_many :related_assets, :dependent => :destroy
   has_many :pages, :through => :related_assets
 
-  scope :images,    -> { where(:upload_content_type => ["image/gif", "image/jpeg", "image/png", "image/webp"]) }
+  scope :images,    -> { where(:upload_content_type => IMAGE_CONTENT_TYPES) }
   scope :documents, -> { where(:upload_content_type => ["application/pdf", "text/plain", "text/rtf"]) }
   scope :audio,     -> { where(:upload_content_type => ["audio/mpeg", "audio/x-m4a", "audio/wav", "audio/x-wav"]) }
 
+  validates :license_key, inclusion: { in: -> { AssetLicense.keys } }, allow_blank: true
+
+  def image?
+    IMAGE_CONTENT_TYPES.include?(upload_content_type)
+  end
 end