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/components/BlogOverview.svelte

@@ -8,6 +8,7 @@
   import { postHref } from "$lib/blog-utils";
   import { t as tStatic, T } from "$lib/translations";
   import type { Translations } from "$lib/translations";
+  import { env } from "$env/dynamic/public";
 
   interface TagEntry {
     _slug?: string;
@@ -113,6 +114,45 @@
   }
 
   const totalCount = $derived(totalPosts);
+
+  // Bulk-fetch comment counts for the visible posts. Uses a single
+  // `/api/comments/counts` request keyed on the post slug. Failures keep the
+  // map empty so cards just don't render the badge.
+  let commentCounts = $state<Record<string, number>>({});
+
+  function postSlug(p: PostEntry): string {
+    const raw = (p as { slug?: string; _slug?: string }).slug ?? p._slug ?? "";
+    return raw.replace(/^\//, "").trim();
+  }
+
+  $effect(() => {
+    const slugs = posts.map(postSlug).filter(Boolean);
+    if (slugs.length === 0) {
+      commentCounts = {};
+      return;
+    }
+    const cmsBase = (
+      env.PUBLIC_CMS_URL ||
+      import.meta.env.PUBLIC_CMS_URL ||
+      ""
+    ).replace(/\/$/, "");
+    if (!cmsBase) return;
+    const url = new URL(`${cmsBase}/api/comments/counts`);
+    url.searchParams.set("ids", slugs.join(","));
+    let cancelled = false;
+    fetch(url.toString())
+      .then((r) => (r.ok ? r.json() : { counts: {} }))
+      .then((d) => {
+        if (cancelled) return;
+        commentCounts = (d?.counts ?? {}) as Record<string, number>;
+      })
+      .catch(() => {
+        if (!cancelled) commentCounts = {};
+      });
+    return () => {
+      cancelled = true;
+    };
+  });
 </script>
 
 <div class="blog-overview">
@@ -258,6 +298,7 @@
           post={post}
           href={postHref(post)}
           tagBase={`${basePath}/tag`}
+          commentCount={commentCounts[postSlug(post)] ?? null}
         />
       {/each}
     </div>