feat(blog): comment count badge on post overview

PostCard now shows a small comment-count pill in the top-left of the
image when count > 0. BlogOverview bulk-fetches counts for the visible
posts via the new `/api/comments/counts?ids=…` endpoint and passes
them down. One request per page render, regardless of card count.

- New helper `getCommentCounts()` in `cms.ts`
- `BlogOverview.svelte` derives slugs from posts, fires bulk fetch
  in `$effect`, hands the result map to `PostCard`
- `PostCard.svelte` accepts `commentCount?: number | null`, renders
  pill only when value is a positive number

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

src/lib/cms.ts

@@ -693,3 +693,34 @@ export async function getCalendarItems(
     return data.items ?? [];
   });
 }
+
+/**
+ * Bulk-fetches approved comment counts for many page_ids in one request.
+ * Hits the comment plugin's `/api/comments/counts` endpoint. Returns a map
+ * `{ pageId → count }` with `0` for any id missing in the response.
+ *
+ * The comment-plugin endpoints live behind a different env (`PUBLIC_CMS_URL`)
+ * and may be on a different origin in dev. SSR-side calls always hit it
+ * directly; the cookie sync of the visitor session does NOT happen here
+ * (counts are public anyway).
+ */
+export async function getCommentCounts(
+  pageIds: string[],
+  options?: { environment?: string },
+): Promise<Record<string, number>> {
+  if (pageIds.length === 0) return {};
+  const base = getBaseUrl();
+  const url = new URL(`${base}/api/comments/counts`);
+  url.searchParams.set("ids", pageIds.join(","));
+  if (options?.environment) {
+    url.searchParams.set("_environment", options.environment);
+  }
+  try {
+    const res = await fetch(url.toString());
+    if (!res.ok) return {};
+    const data = (await res.json()) as { counts?: Record<string, number> };
+    return data.counts ?? {};
+  } catch {
+    return {};
+  }
+}