fix: thumbnail-first + megapixel guard to prevent image OOM in PDF/DOCX

Previous draft()+convert()+thumbnail() ordering still caused OOM for
non-JPEG formats (PNG/HEIC) because draft() is a no-op for those formats,
and convert('RGB') forces a full pixel decode regardless.

Replace with thumbnail()-first ordering:
- thumbnail() internally calls draft() for JPEG before decoding
- thumbnail() performs in-place resize without allocating a second
  full-resolution buffer
- convert('RGB') is then called on the already-small image (safe for
  any format)

Add _COMPRESS_IMAGE_MAX_MEGAPIXELS guard (20 MP): read image dimensions
from headers only (no pixel decode) and return None for images that
exceed the limit. This prevents OOM even for pathologically large files
where draft() provides no benefit (e.g. PNG, TIFF, HEIC).

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

README.md

@@ -7,7 +7,7 @@ Nakon što je gitea webhook i testirana skripta za automatsko povlačenje promje
 ```bash
 cd /opt/erp/app
 git fetch origin
-git reset --hard origin/main
+git reset --hard origin/main ili git pull --ff-only origin main
 
 # rebuild backend + worker + beat + frontend
 docker compose -f docker-compose.yml -f docker-compose.prod.yml build backend worker beat frontend