feat: chat — photo attachments

- Migration 073: attachment_url column in chat_messages
- upload.ts: add 'chat' folder to ALLOWED_FOLDERS
- chat.ts: accept attachment_url in send message, return it in GET messages
- api.ts: attachmentUrl in ChatMessage type, uploadImage() with raw FormData fetch
- ChatWidget: paperclip button, file preview thumbnail with remove, image display
  inline in message bubbles (clickable, opens original), send enabled with image only

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/routes/upload.ts

@@ -7,7 +7,7 @@ import { pipeline } from 'stream/promises'
 const UPLOADS_DIR = process.env.UPLOADS_DIR ?? join(process.cwd(), 'uploads')
 const CDN_HOST    = process.env.CDN_HOST ?? 'cdn.hotelsync.ru'
 
-const ALLOWED_FOLDERS = ['categories', 'rooms', 'hotels', 'guests', 'tasks'] as const
+const ALLOWED_FOLDERS = ['categories', 'rooms', 'hotels', 'guests', 'tasks', 'chat'] as const
 type UploadFolder = typeof ALLOWED_FOLDERS[number]
 
 const ALLOWED_TYPES = ['image/jpeg', 'image/png', 'image/webp', 'image/gif']