perf(cms): batch layout bootstrap, sparse fieldsets, depth caps

Adopt RustyCMS' new resolve semantics (_depth, _fields,
POST /_batch) where they pay off:

- +layout.server.ts: 4 sequential GETs (navHeader, navSocial,
  footer, page_config) collapsed into one batchFetch() call.
  Legacy "default" page_config slug stays as fallback Single-
  Batch only when the canonical slug is missing.
- New getPageStubs() / getPostStubs() use _fields= so the
  Slug→Label/Slug→URL maps don't pull body/rows. Typically
  10-50× smaller payload, eligible for the same TTL cache.
- loadPostsList() now requests posts with _depth=2 and a
  sparse fieldset that keeps card-level fields plus
  postImage.* and postTag.{name,icon,color,_slug}. Body
  Markdown stays out of the listing entirely.
- Page + post detail routes set _depth=3 as a cycle/blast-radius
  cap (page → block → image → img reaches in three levels).
- cms.ts: ContentGetOptions gains depth/fields, getPageBySlug /
  getPostBySlug / getPosts thread them through (cache key
  extended). New batchFetch() / batchData<T>() helpers.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/lib/blog-utils.ts

@@ -255,8 +255,31 @@ export async function loadPostsList(opts: {
   let cmsError: string | null = null;
 
   try {
+    // Sparse fieldset für die Listen-Ansicht: Body/Rows brauchen wir hier
+    // nicht, nur Karten-Felder + image + tag-Metadaten. _depth=2 reicht
+    // (post → postImage → img-Sidecar; post → postTag → name/icon/color).
     [posts, tags] = await Promise.all([
-      getPosts({ resolve: ["postImage"] }),
+      getPosts({
+        resolve: ["postImage", "postTag"],
+        depth: 2,
+        fields: [
+          "_slug",
+          "slug",
+          "headline",
+          "linkName",
+          "subheadline",
+          "excerpt",
+          "created",
+          "isEvent",
+          "eventDate",
+          "hideFromListing",
+          "postImage.*",
+          "postTag.name",
+          "postTag.icon",
+          "postTag.color",
+          "postTag._slug",
+        ],
+      }),
       getTags(),
     ]);
     posts = filterHiddenPosts(sortPostsByDate(posts));