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/schema.ts

@@ -85,6 +85,7 @@ export const attachments = sqliteTable('attachments', {
   originalName: text('original_name').notNull(),
   mimetype: text('mimetype').notNull(),
   size: integer('size').notNull(),
+  thumbnailFilename: text('thumbnail_filename'),
   createdAt: integer('created_at').notNull(),
 });