Add PDF/HEIC/image upload support and improve receipt extraction

- Add backend image conversion endpoint (POST /api/convert-image) supporting
  PDF, HEIC, PNG, WebP via Pillow, PyMuPDF, and pillow-heif
- Add separate "Upload file" button in UI while keeping camera-first behavior
  for the photo area and + button
- Improve Haiku extraction prompt for hotel receipts (parenthesized total)
- Increase max image resolution from 1024px to 2048px for better OCR accuracy
- Add libheif-dev system dependency in Dockerfile

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

requirements.txt

@@ -1,2 +1,5 @@
 requests
 openpyxl
+Pillow
+PyMuPDF
+pillow-heif