perf: memoize Store#all with mtime-based invalidation

Cache the parsed post list per Store instance, invalidated on save,
delete, or content_dir mtime change. Eliminates re-parsing every .md
file on every API and admin request.

lib/volumen/store.rb

@@ -16,10 +16,16 @@ module Volumen
     def initialize(content_dir, default_lang: nil)
       @content_dir = File.expand_path(content_dir.to_s)
       @default_lang = default_lang
+      @all = nil
     end
 
     def all
-      markdown_files.filter_map { |path| load_post(path) }
+      mtime = Dir.exist?(@content_dir) ? File.mtime(@content_dir) : nil
+      @all = nil if mtime != @all_mtime
+      @all ||= begin
+        @all_mtime = mtime
+        markdown_files.filter_map { |path| load_post(path) }
+      end
     end
 
     def find(slug, lang: nil)
@@ -33,12 +39,16 @@ module Volumen
       FileUtils.mkdir_p(File.dirname(target))
       File.write(target, post.to_file)
       post.path = target
+      @all = nil
       post
     end
 
     def delete(slug, lang: nil)
       post = find(slug, lang: lang)
-      File.delete(post.path) if post&.path
+      if post&.path
+        File.delete(post.path)
+        @all = nil
+      end
       post
     end