feat(post): add h-entry microformat + tag-name keywords fallback

Microformats:
- Wrap the post <article> in h-entry with p-name on the headline,
  p-summary on the subheadline (or sr-only fallback from excerpt),
  e-content on the rendered body, dt-published on the created date
  (hidden time element) and an invisible p-author h-card. Tag names
  are mirrored as hidden p-category entries so crawlers see them
  even though the visual Tags component already links them.
- itemscope/itemtype maps to schema.org BlogPosting (or Event when
  `isEvent`), aligning the invisible microdata with the existing
  JSON-LD.

Keywords:
- When post.seoKeywords is empty, fall back to the comma-joined tag
  names so <meta name="keywords"> still ships something meaningful.

src/routes/post/[slug]/+page.server.ts

@@ -232,7 +232,17 @@ export const load: PageServerLoad = async ({ params, locals, url, setHeaders })
     seoDescription: postDescription,
     socialImage: postSocialImageAbs ?? postImageUrl ?? undefined,
     robots: (post as { seoMetaRobots?: string }).seoMetaRobots ?? undefined,
-    keywords: (post as { seoKeywords?: string }).seoKeywords ?? undefined,
+    keywords: (() => {
+      const explicit = ((post as { seoKeywords?: string }).seoKeywords ?? '').trim();
+      if (explicit) return explicit;
+      // Fallback: join tag names so published posts without explicit keywords
+      // still ship a meaningful <meta name="keywords">.
+      const tags = (post as { postTag?: Array<{ name?: string }> }).postTag ?? [];
+      const names = tags
+        .map((t) => (typeof t?.name === 'string' ? t.name.trim() : ''))
+        .filter(Boolean);
+      return names.length > 0 ? names.join(', ') : undefined;
+    })(),
     jsonLd,
     breadcrumbItems: [
       { href: '/', label: 'Start' },

src/routes/post/[slug]/+page.svelte

@@ -72,21 +72,36 @@
   </div>
 </div>
 
-<article class="mt-8">
+<article
+  class="mt-8 h-entry"
+  itemscope
+  itemtype={data.isEvent ? 'https://schema.org/Event' : 'https://schema.org/BlogPosting'}
+>
+  <a class="u-url" href="#" aria-hidden="true" hidden></a>
+  {#if data.post._created || data.post.created}
+    <time
+      class="dt-published"
+      datetime={String(data.post.created ?? data.post._created ?? '')}
+      hidden
+    >{data.post.created ?? data.post._created}</time>
+  {/if}
   {#if data.contentHeadlineHtml}
-    <div class="content markdown max-w-none prose prose-zinc">
+    <div class="content markdown max-w-none prose prose-zinc p-name">
       {@html data.contentHeadlineHtml}
     </div>
   {:else if data.post.headline || data.post.subheadline}
     <header class="content markdown max-w-none prose prose-zinc mb-4">
       {#if data.post.headline}
-        <h1>{data.post.headline}</h1>
+        <h1 class="p-name">{data.post.headline}</h1>
       {/if}
       {#if data.post.subheadline}
-        <p class="text-zinc-600 lead">{data.post.subheadline}</p>
+        <p class="text-zinc-600 lead p-summary">{data.post.subheadline}</p>
       {/if}
     </header>
   {/if}
+  {#if !data.post.subheadline && data.post.excerpt}
+    <p class="p-summary sr-only">{data.post.excerpt}</p>
+  {/if}
   {#if data.isEvent && (data.eventDateLabel || data.eventLocationText)}
     <div class="mb-6">
       <EventBadges
@@ -97,13 +112,25 @@
     </div>
   {/if}
   {#if data.contentBodyHtml}
-    <div class="content markdown max-w-none prose prose-zinc">
+    <div class="content markdown max-w-none prose prose-zinc e-content">
       {@html data.contentBodyHtml}
     </div>
   {/if}
   {#if data.hasRowContent}
     <ContentRows layout={data.post} translations={data.translations} />
   {/if}
+  {#if (data.post.postTag ?? []).length > 0}
+    <div hidden>
+      {#each data.post.postTag ?? [] as tag}
+        {#if tag && typeof tag === 'object' && 'name' in tag && tag.name}
+          <span class="p-category">{tag.name}</span>
+        {/if}
+      {/each}
+    </div>
+  {/if}
+  <div class="p-author h-card" hidden>
+    <span class="p-name">{data.post.author ?? 'Bürgerinitiative Vachdorf'}</span>
+  </div>
 </article>
 
 {#if data.isEvent && (data.mapEmbedUrl || data.mapLinkUrl)}