perf: invalidate Store cache on individual file mtime changes

Previously the store only checked the content directory's own mtime,
which does not change when an existing file is modified in place. Now
track the newest mtime across all .md files as well, so edits picked
up from disk (manual changes, git pull) are detected without a
restart.

lib/volumen/store.rb

@@ -20,10 +20,12 @@ module Volumen
     end
 
     def all
-      mtime = Dir.exist?(@content_dir) ? File.mtime(@content_dir) : nil
+      current_dir_mtime = Dir.exist?(@content_dir) ? File.mtime(@content_dir) : nil
-      @all = nil if mtime != @all_mtime
+      current_files_mtime = newest_file_mtime
+      @all = nil if current_dir_mtime != @all_dir_mtime || current_files_mtime != @all_files_mtime
       @all ||= begin
-        @all_mtime = mtime
+        @all_dir_mtime = current_dir_mtime
+        @all_files_mtime = current_files_mtime
         markdown_files.filter_map { |path| load_post(path) }
       end
     end
@@ -79,6 +81,15 @@ module Volumen
       Dir.glob(File.join(@content_dir, "**", "*.md"))
     end
 
+    def newest_file_mtime
+      files = markdown_files
+      return nil if files.empty?
+
+      files.map { |path| File.mtime(path) }.max
+    rescue SystemCallError
+      nil
+    end
+
     def load_post(path)
       post = Post.parse(File.read(path))
       post.metadata["slug"] ||= File.basename(path, ".md")