images: bust CF cache for the de-watermarked art; add the dewatermark tool

public/ assets are edge-cached under non-hashed URLs, so the scrubbed covers
and heroes were still being served stale. Project covers carried no cache-bust
at all - added ?v=3 to both cover references and bumped the blog heroes from
v2 to v3. Adds scripts/dewatermark.mjs implementing the documented approach
(hard-coded mark centre, feathered disc, border-mean seed, Jacobi diffusion)
rather than the ffmpeg delogo used in the previous pass.

src/components/PostList.astro

@@ -17,7 +17,7 @@ const { posts } = Astro.props;
         <a class="post__link" href={`/blog/${post.id}/`}>
           {post.data.hero && (
             <div class="post__thumb">
-              <img src={`${post.data.hero}?v=2`} alt="" width="1600" height="1073" loading="lazy" decoding="async" />
+              <img src={`${post.data.hero}?v=3`} alt="" width="1600" height="1073" loading="lazy" decoding="async" />
             </div>
           )}
           <div class="post__card-body">

src/components/ProjectCard.astro

@@ -20,7 +20,7 @@ const dossierNo = String(index).padStart(3, "0");
     {p.cover
       ? <img
           class="dossier__cover"
-          src={`/covers/thumb/${p.cover}.webp`}
+          src={`/covers/thumb/${p.cover}.webp?v=3`}
           alt=""
           width="640"
           height="429"

src/pages/blog/[slug].astro

@@ -43,7 +43,7 @@ const { Content } = await render(entry);
       {entry.data.hero && (
         <figure class="post__hero" data-reveal>
           <img
-            src={`${entry.data.hero}?v=2`}
+            src={`${entry.data.hero}?v=3`}
             alt={entry.data.heroAlt ?? entry.data.title}
             width="1600"
             height="1073"

src/pages/blog/index.astro

@@ -56,7 +56,7 @@ const featuredSeries = series[featured.id];
         {featured.data.hero && (
           <div class="featured__art" aria-hidden="true">
             <img
-              src={`${featured.data.hero}?v=2`}
+              src={`${featured.data.hero}?v=3`}
               alt=""
               width="1600"
               height="1073"

src/pages/projects/[slug].astro

@@ -22,7 +22,7 @@ const { Content } = await render(entry);
       {p.cover && (
         <figure class="case__cover" data-reveal>
           <img
-            src={`/covers/${p.cover}.webp`}
+            src={`/covers/${p.cover}.webp?v=3`}
             alt={`${p.title} - cover art`}
             width="1600"
             height="1073"