fix: post thumbnail from linked design artefact, not old PostAttachments

- Post.thumbnail_url now synced from linked design artefact's first attachment
- syncPostThumbnail() called on: artefact attachment upload, artefact link/unlink
- Removed old PostAttachments-based thumbMap from GET /posts and GET /campaigns/:id/posts
- Added thumbnail_url to Posts TEXT_COLUMNS
- Caption link picker filters by copy_type='caption', body by copy_type='body'

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

server/post-composition.js

@@ -96,4 +96,31 @@ function computeStage(composition) {
   return 'copy';
 }
 
-module.exports = { getPostComposition, computeStage };
+// Sync the post's thumbnail_url from its linked design artefact
+async function syncPostThumbnail(postId) {
+  try {
+    const artefacts = await nocodb.list('Artefacts', {
+      where: `(post_id,eq,${postId})`, limit: 100,
+    });
+    const design = artefacts.find(a => (a.type || 'design') === 'design');
+    let thumb = null;
+    if (design) {
+      thumb = design.thumbnail_url || null;
+      if (!thumb) {
+        const versions = await nocodb.list('ArtefactVersions', { where: `(artefact_id,eq,${design.Id})`, sort: '-version_number', limit: 1 });
+        if (versions.length > 0) {
+          const attachments = await nocodb.list('ArtefactAttachments', { where: `(version_id,eq,${versions[0].Id})`, limit: 1 });
+          if (attachments.length > 0) {
+            const att = attachments[0];
+            thumb = att.drive_url || (att.filename ? `/api/uploads/${att.filename}` : null);
+          }
+        }
+      }
+    }
+    await nocodb.update('Posts', Number(postId), { thumbnail_url: thumb || null });
+  } catch (e) {
+    console.error('syncPostThumbnail error:', e);
+  }
+}
+
+module.exports = { getPostComposition, computeStage, syncPostThumbnail };