docs(BlockEditor): document mediaSlug media library link and add server helpers

- update image block schema in README table to include `mediaSlug?` and clarify fields
- add "Liaison avec la médiathèque" section documenting mediaSlug behavior, read-only alt/caption, and internal `_` fields
- document new server helpers (`normalizeImageBlocks`, `enrichBlocksWithMedia`, `syncBlockImageReferences`) with usage examples
- add `block_image` field convention to media feature README with cross-references
- implement `mediaLink.server.js` with the three server-side helpers
- store `mediaSlug` on image block at insertion time in `Image.client.js`
- persist `mediaSlug` through clipboard paste/duplicate in `clipboard.js`
- export `mediaLink` entry point in `package.json` exports map

src/shared/components/BlockEditor/inline/clipboard.js

@@ -15,6 +15,16 @@ import { inlineFromText, inlineToPlainText, normalize } from './types.js';
 
 const HEADING_RE = /^heading_([1-6])$/;
 const BLOCK_TAG_RE = /^(P|H[1-6]|UL|OL|BLOCKQUOTE|PRE|HR|FIGURE|DIV|TABLE)$/;
+const MEDIA_FILE_URL_RE = /^\/zen\/api\/media\/file\/([^/?#]+)$/;
+
+// Quand une image collée pointe sur la médiathèque interne, on dérive le
+// slug pour reconstituer le lien `mediaSlug` (un copier-coller ne traverse
+// pas les champs cachés). Pour les URL externes, retourne null.
+function imageSlugFromSrc(src) {
+  if (typeof src !== 'string') return null;
+  const m = MEDIA_FILE_URL_RE.exec(src);
+  return m ? m[1] : null;
+}
 
 // Block[] → HTML string. Regroupe les listes consécutives sous un seul
 // <ul>/<ol>. Les blocs inconnus deviennent un <p> au texte aplati.
@@ -98,9 +108,15 @@ function blockToElement(block) {
         : align === 'left' ? 'text-align:left'
         : 'text-align:right');
     }
+    // Quand le bloc est lié à un média (`mediaSlug`), alt/caption sont
+    // résolus côté serveur (`_resolvedAlt` / `_resolvedCaption`) ou
+    // captés à l'insertion (`_mediaAlt` / `_mediaCaption`). Pour les URL
+    // externes, on retombe sur les champs locaux du bloc.
+    const altText = block._resolvedAlt ?? block._mediaAlt ?? block.alt ?? '';
+    const captionText = block._resolvedCaption ?? block._mediaCaption ?? block.caption ?? '';
     const img = document.createElement('img');
     img.setAttribute('src', block.src || '');
-    if (block.alt) img.setAttribute('alt', block.alt);
+    if (altText) img.setAttribute('alt', altText);
     if (align === 'full') img.setAttribute('width', '100%');
     let imgHost = img;
     if (block.href) {
@@ -114,9 +130,9 @@ function blockToElement(block) {
       imgHost = a;
     }
     fig.appendChild(imgHost);
-    if (block.caption) {
+    if (captionText) {
       const cap = document.createElement('figcaption');
-      cap.textContent = block.caption;
+      cap.textContent = captionText;
       fig.appendChild(cap);
     }
     return fig;
@@ -133,7 +149,11 @@ export function blocksToPlainText(blocks) {
   return blocks
     .map(b => {
       if (b.type === 'divider') return '---';
-      if (b.type === 'image') return b.alt || b.caption || '';
+      if (b.type === 'image') {
+        const alt = b._resolvedAlt ?? b._mediaAlt ?? b.alt ?? '';
+        const cap = b._resolvedCaption ?? b._mediaCaption ?? b.caption ?? '';
+        return alt || cap || '';
+      }
       return inlineToPlainText(b.content ?? []);
     })
     .join('\n');
@@ -296,15 +316,21 @@ function parseChildren(node, out) {
         const align = ['left', 'center', 'right', 'full'].includes(dataAlign) ? dataAlign : 'center';
         const href = linkEl?.getAttribute('href') || '';
         const newTab = !!href && linkEl?.getAttribute('target') === '_blank';
+        const src = img.getAttribute('src') || '';
+        const slug = imageSlugFromSrc(src);
+        const altText = img.getAttribute('alt') || '';
+        const captionText = cap?.textContent?.trim() || '';
         out.push({
           id: newBlockId(),
           type: 'image',
-          src: img.getAttribute('src') || '',
-          alt: img.getAttribute('alt') || '',
-          caption: cap?.textContent?.trim() || '',
+          src,
+          mediaSlug: slug || '',
+          alt: slug ? '' : altText,
+          caption: slug ? '' : captionText,
           align,
           href,
           newTab,
+          ...(slug ? { _mediaAlt: altText, _mediaCaption: captionText } : null),
         });
       }
       continue;
@@ -312,15 +338,20 @@ function parseChildren(node, out) {
 
     if (tag === 'IMG') {
       flush();
+      const src = child.getAttribute('src') || '';
+      const slug = imageSlugFromSrc(src);
+      const altText = child.getAttribute('alt') || '';
       out.push({
         id: newBlockId(),
         type: 'image',
-        src: child.getAttribute('src') || '',
-        alt: child.getAttribute('alt') || '',
+        src,
+        mediaSlug: slug || '',
+        alt: slug ? '' : altText,
         caption: '',
         align: 'center',
         href: '',
         newTab: false,
+        ...(slug ? { _mediaAlt: altText } : null),
       });
       continue;
     }