feat: add scheduled publishing with publish_at frontmatter field

Posts with a future publish_at date are hidden from the public API
and feeds, same as drafts. The admin form exposes a Publish at field.

lib/volumen/post.rb

@@ -56,6 +56,24 @@ module Volumen
       metadata["cover"]
     end
 
+    def publish_at
+      metadata["publish_at"]
+    end
+
+    def scheduled?
+      value = publish_at
+      return false if value.nil?
+
+      date = case value
+             when Date then value
+             when Time then value.to_date
+             else Date.iso8601(value.to_s)
+             end
+      date > Date.today
+    rescue ArgumentError
+      false
+    end
+
     def reading_time
       words = body.split(/\s+/).count { |word| !word.empty? }
       [(words / 200.0).ceil, 1].max

lib/volumen/server.rb

@@ -79,7 +79,8 @@ module Volumen
     end
 
     def published_posts
-      store.all.reject(&:draft?).sort_by { |post| post.date_string || "" }.reverse
+      store.all.reject { |post| post.draft? || post.scheduled? }
+               .sort_by { |post| post.date_string || "" }.reverse
     end
 
     def site_payload
@@ -369,6 +370,7 @@ module Volumen
         "lang" => presence(http_params["lang"]),
         "author" => presence(http_params["author"]),
         "date" => parse_date(http_params["date"]),
+        "publish_at" => parse_date(http_params["publish_at"]),
         "tags" => parse_tags(http_params["tags"]),
         "excerpt" => presence(http_params["excerpt"]),
         "cover" => presence(http_params["cover"]),

lib/volumen/web/views/form.erb

@@ -17,6 +17,11 @@
     <label>Language <input type="text" name="lang" value="<%= h(@post.lang) %>"></label>
     <label>Author <input type="text" name="author" value="<%= h(@post.author) %>"></label>
     <label>Date <input type="date" name="date" value="<%= h(@post.date_string) %>"></label>
+    <label>Publish at
+      <input type="date" name="publish_at"
+             value="<%= @post.publish_at.is_a?(Date) ? @post.publish_at.strftime("%Y-%m-%d") : "" %>"
+             placeholder="leave empty for immediate">
+    </label>
     <label>Tags
       <input type="text" name="tags" value="<%= h(@post.tags.join(", ")) %>"
              placeholder="comma, separated">