Use no-store for docs: epoch mtimes make revalidation serve stale

modules/docs.nix

@@ -30,11 +30,12 @@ in
     enable = true;
     virtualHosts.":${toString port}".extraConfig = ''
       root * ${site}
-      # mdBook doesn't fingerprint asset filenames, so a browser left to its own
-      # heuristics caches the HTML and serves stale docs after a redeploy. Force a
-      # conditional request on every load; file_server's ETag/Last-Modified makes
-      # the unchanged case a cheap 304 over the mesh.
-      header Cache-Control "no-cache"
+      # mdBook doesn't fingerprint asset filenames, and every file in the Nix
+      # store carries an epoch (1970) mtime, so file_server's only validator never
+      # changes across redeploys — conditional requests would 304 forever and pin
+      # browsers to stale docs. no-store sidesteps caching entirely; pages are tiny
+      # and mesh-only, so refetching on each load is free.
+      header Cache-Control "no-store"
       file_server
     '';
   };