feat: image optimization — client-side resize + server-side thumbnails

Avatars/banners now resize to max 512px/1920px and convert to WebP before
upload (zero server cost). Chat image uploads generate an 800px-wide WebP
thumbnail via Sharp; the feed shows the thumbnail, click opens the full-res
original. Adds lazy loading to avatars. Federation-compatible: remote
instances without this feature fall back gracefully.

packages/server/src/utils/fileCleanup.ts

@@ -1,11 +1,13 @@
 import fs from 'fs';
 import path from 'path';
 import { config } from '../config.js';
+import { thumbFilename } from './thumbnail.js';
 
 /**
  * Delete a single uploaded file by its stored filename.
  * Uses path.basename() to prevent directory traversal attacks.
  * Tolerates ENOENT (file already gone) but logs other errors.
+ * Also attempts to delete the corresponding thumbnail if one exists.
  */
 export function deleteUploadFile(filename: string): void {
   const safeName = path.basename(filename);
@@ -17,6 +19,15 @@ export function deleteUploadFile(filename: string): void {
       console.error(`Failed to delete upload file ${safeName}:`, err.message);
     }
   }
+
+  // Also attempt to delete the thumbnail variant
+  const thumbName = thumbFilename(safeName);
+  const thumbPath = path.join(config.uploadDir, thumbName);
+  try {
+    fs.unlinkSync(thumbPath);
+  } catch {
+    // Thumbnail may not exist — that's fine
+  }
 }
 
 /**