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/migrations/073_chat_attachments.sql

@@ -0,0 +1 @@
+ALTER TABLE chat_messages ADD COLUMN IF NOT EXISTS attachment_url TEXT;