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/db/migrate.ts

@@ -44,6 +44,12 @@ export function runMigrations(db: Database.Database): void {
         { name: 'closed', type: 'INTEGER DEFAULT 0' }
       ]
     },
+    {
+      name: 'attachments',
+      columns: [
+        { name: 'thumbnail_filename', type: 'TEXT' }
+      ]
+    },
     {
       name: 'dm_channels',
       columns: [