feat(manage): add /manage pages to edit caption and date

/manage lists every photo; /manage/edit/{slug} rewrites the photo's
.toml sidecar, which triggers a rebuild. The pages sit behind the same
htpasswd as /upload and are not linked from the public site.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

module.nix

@@ -94,7 +94,7 @@ in {
       htpasswdFile = lib.mkOption {
         type        = lib.types.path;
         example     = "/etc/nginx/.htpasswd-gallery";
-        description = "Path to htpasswd file protecting the /upload endpoint.";
+        description = "Path to htpasswd file protecting the /upload and /manage endpoints.";
       };
 
       enableACME = lib.mkOption {
@@ -184,6 +184,12 @@ in {
             basicAuthFile = cfg.nginx.htpasswdFile;
             extraConfig   = "client_max_body_size 20M;";
           };
+
+          locations."/manage" = {
+            proxyPass     = "http://127.0.0.1:${toString cfg.port}";
+            basicAuth     = "gallery";
+            basicAuthFile = cfg.nginx.htpasswdFile;
+          };
         };
       };
     })