Add Server/Last-Modified headers and a {dates} dateline tag

An http middleware sets 'Server: pagerite' (dropping uvicorn's versioned
default) and content pages return Last-Modified from Node.modified. In
markdown, a {dates} line expands to the article's published/updated
dateline (updated shown only when it falls on a later day); the editor
preview resolves it for pages that exist, unsaved pages show it literal.
Demonstrated in the long-read seed page.

frontend/src/assets/pagerite.css

@@ -777,6 +777,14 @@ body.editing:has(.multicol) img.wide:not(figure img) {
   list-style: none;
 }
 
+/* Published/updated line, expanded from the {dates} tag in the markdown
+   (typically placed right after the article's h1). */
+.dateline {
+  color: var(--muted);
+  font-size: 0.85rem;
+  text-align: left;
+}
+
 .footnote {
   font-size: 0.85rem;
   color: var(--muted);