feat(api): POST /api/cache/clear + flush images on asset webhooks

Two changes:
- New POST /api/cache/clear (auth: REVALIDATE_TOKEN). Wipes the
  TTL content cache, the image disk + memory cache, and forwards to
  the RustyCMS /api/transform/cache/invalidate endpoint when an
  API key is configured. Single trigger to flush everything.
- /api/revalidate now also calls clearImageCache() on asset.* events
  so a re-upload (focal change, alt edit, replacement) doesn't keep
  serving the previous transformed image from disk.

clearImageCache() in image-cache.server.ts wipes the in-memory map
and removes every file under IMAGE_CACHE_DIR.

src/lib/image-cache.server.ts

@@ -218,3 +218,27 @@ export function resolveExtension(params: ImageTransformParams, contentType?: str
   if (contentType) return extForContentType(contentType);
   return 'jpg';
 }
+
+/**
+ * Wipe in-memory + on-disk image cache. Returns counts cleared.
+ */
+export async function clearImageCache(): Promise<{ memory: number; disk: number }> {
+  const memory = memoryCache.size;
+  memoryCache.clear();
+  let disk = 0;
+  try {
+    const dir = ensureCacheDir();
+    const files = await readdir(dir);
+    for (const f of files) {
+      try {
+        await unlink(join(dir, f));
+        disk++;
+      } catch {
+        /* ignore */
+      }
+    }
+  } catch {
+    /* ignore */
+  }
+  return { memory, disk };
+}