feat(images): cache-bust /cms-images URLs by post._updated

Browser caches /cms-images responses immutably for a year. When the
editor changes a focal point or alt text, the underlying transform
output changes too, but the URL stays the same — so the browser keeps
serving the previous bytes. Manual hard-reloads and /api/cache/clear
help once but not for visitors who already have the file.

- RustyImageTransformParams gains an optional `version` that
  serializes as `_v` in the /cms-images URL.
- RustyImage.svelte takes a matching `version` prop and forwards it
  through `baseParams`, so every <picture> source carries the tag.
- The post thumbnail passes `post._updated` (truncated to seconds)
  as version. Each save bumps `_updated`, the URL changes, browsers
  re-fetch.

`_v` is intentionally not consumed by `/cms-images` — its sole job is
to break the browser cache key. The server-side image-cache and the
RustyCMS transform cache key both ignore it, so disk usage doesn't
balloon when the version tag changes.

src/lib/components/RustyImage.svelte

@@ -37,6 +37,8 @@
     /** JPEG/AVIF/WebP-Qualität. Default 75. */
     quality?: number;
     fit?: "cover" | "contain" | "fill";
+    /** Cache-Bust-Tag (z.B. post._updated) → `_v`-Param, bricht Browser-HTTP-Cache. */
+    version?: string;
   }
 
   let {
@@ -56,6 +58,7 @@
     formats = ["webp", "jpeg"],
     quality = 75,
     fit = "cover",
+    version,
   }: Props = $props();
 
   function parseAr(s: string | undefined): number | null {
@@ -89,6 +92,7 @@
       ...(arString && fit !== "contain"
         ? { height: Math.round(w / (arValue ?? 1)) }
         : {}),
+      ...(version ? { version } : {}),
     };
   }