feat(post): add cover caption metadata

lib/volumen/post.rb

@@ -62,6 +62,10 @@ module Volumen
       metadata["cover_alt"]
     end
 
+    def cover_caption
+      metadata["cover_caption"]
+    end
+
     def fediverse_creator
       value = metadata["fediverse_creator"]
       return nil if value.nil?
@@ -133,6 +137,7 @@ module Volumen
         "fediverse_creator" => fediverse_creator,
         "cover" => cover,
         "cover_alt" => cover_alt,
+        "cover_caption" => cover_caption,
         "reading_time" => reading_time,
         "translations" => translations,
         "url" => "/api/volumen/posts/#{slug}"

lib/volumen/server.rb

@@ -395,6 +395,7 @@ module Volumen
         "excerpt" => presence(http_params["excerpt"]),
         "cover" => presence(http_params["cover"]),
         "cover_alt" => presence(http_params["cover_alt"]),
+        "cover_caption" => presence(http_params["cover_caption"]),
         "draft" => http_params["draft"] == "on",
         "all_langs" => http_params["all_langs"] == "on"
       }

lib/volumen/web/views/form.erb

@@ -48,6 +48,8 @@
     </div>
     <input type="text" id="cover-alt-input" name="cover_alt" value="<%= h(@post.cover_alt) %>"
            placeholder="ALT text (describe the image for accessibility)">
+    <input type="text" id="cover-caption-input" name="cover_caption" value="<%= h(@post.cover_caption) %>"
+           placeholder="Caption (e.g. 'Generated by AI', shown in popup)">
     <img id="cover-preview" class="cover-preview" alt="" hidden>
     <input type="file" id="cover-file" accept="image/*" hidden>
   </div>