perf: review-fixes (N+1, waterfalls, sanitize-cache, lazy strommix)

- blog-utils.resolvePostOverviewBlocks: parallel getPostBySlug via
  Promise.all + .catch(null) instead of sequential await loop.
- blog-utils.resolveCalendarBlocks: same fix for calendar items.
- [...slug] page load: SearchableText/Calendar/DeadlineBanner now run
  parallel after tagsMap (was strict waterfall).
- sanitize-blocks: LRU cache keyed on raw html input — sanitize-html is
  ~ms/KB, the same content repeats across pages and cache hits.
- cms.getNormalizedTranslations: WeakMap-memoised the per-request
  Object.entries normalize loop on top of the existing 5-min bundle TTL.
- cms.cached: opt-in dev cache via DEV_CACHE_CMS=1 (default still off).
- rusty-image.ensureTransformedImage: was async-wrapping a sync URL
  builder; made sync, removed awaits at call sites — saves microtasks.
- BlockRenderer: lazy-load StrommixBlock (~220 KB) via dynamic import
  in the template — pages without the live-strommix widget no longer
  ship its bundle.

Also: enabled Caddy zstd+gzip on windwiderstand.de vhost (server-side
edit; verified `content-encoding: zstd` on response).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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

@@ -51,11 +51,18 @@ export const load: PageServerLoad = async ({ params, locals, fetch, url, setHead
     error(404, 'Seite nicht gefunden');
   }
 
+  // resolveContentImages baut nur URLs zusammen (kein I/O) und mutiert den
+  // Tree — vor den Block-Resolvern erledigen, die danach gegen mutierte
+  // Blocks arbeiten.
   await resolveContentImages(page);
+  // PostOverview liefert tagsMap, den SearchableText braucht. Calendar +
+  // DeadlineBanner sind unabhängig → parallel mit SearchableText laufen.
   const tagsMap = await resolvePostOverviewBlocks(page);
-  await resolveSearchableTextBlocks(page, tagsMap);
-  await resolveCalendarBlocks(page);
-  await resolveDeadlineBannerBlocks(page);
+  await Promise.all([
+    resolveSearchableTextBlocks(page, tagsMap),
+    resolveCalendarBlocks(page),
+    resolveDeadlineBannerBlocks(page),
+  ]);
   sanitizeBlocks(page);
 
   // Resolve top banner if present