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/components/BlockRenderer.svelte

@@ -18,7 +18,6 @@
   import OpnFormBlock from "./blocks/OpnFormBlock.svelte";
   import InternalComponentBlock from "./blocks/InternalComponentBlock.svelte";
   import DeadlineBannerBlock from "./blocks/DeadlineBannerBlock.svelte";
-  import StrommixBlock from "./blocks/StrommixBlock.svelte";
   import WindkarteBlock from "./blocks/WindkarteBlock.svelte";
   import type { Translations } from "$lib/translations";
   import type {
@@ -93,7 +92,11 @@
 {:else if type === "deadline_banner"}
   <DeadlineBannerBlock block={block as DeadlineBannerBlockData} />
 {:else if type === "live_strommix"}
-  <StrommixBlock block={block as LiveStrommixBlockData} />
+  <!-- Lazy: ~220 KB Block-Bundle nur laden wenn Seite das Widget tatsächlich hat -->
+  {#await import("./blocks/StrommixBlock.svelte") then m}
+    {@const Comp = m.default}
+    <Comp block={block as LiveStrommixBlockData} />
+  {/await}
 {:else if type === "wind_map"}
   <WindkarteBlock block={block as WindMapBlockData} />
 {:else}