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/lib/rusty-image.ts

@@ -108,7 +108,7 @@ export function getTransformUrl(url: string, params: RustyImageTransformParams =
 }
 
 /** Alias für Kompatibilität (z. B. rustyastro / Layout). */
-export async function ensureTransformedImage(url: string, params: RustyImageTransformParams = {}): Promise<string> {
+export function ensureTransformedImage(url: string, params: RustyImageTransformParams = {}): string {
   return getCmsImageUrl(url, params);
 }