Improve PDF generation with proper A4 sizing and borders

- Move PDF border size to settings.py
- Use standard A4 dimensions (210x297mm)
- Fix aspect ratio preservation in layout function
- Use img2pdf's built-in layout function with proper border specification

settings.py

@@ -23,6 +23,9 @@ DEFAULT_PARALLEL_PROCESSES = max(1, multiprocessing.cpu_count() - 1)
 # Can be overridden by environment variable
 PARALLEL_PROCESSES = int(os.getenv('NOTES_CLEANER_PARALLEL_PROCESSES', DEFAULT_PARALLEL_PROCESSES))
 
+# Border size in pixels for the final PDF output
+PDF_BORDER_SIZE = 50
+
 # Optimization settings
 OPTIPNG_OPTIMIZATION_LEVEL = int(os.getenv('NOTES_CLEANER_OPTIPNG_LEVEL', '7'))  # 0-7, higher = better compression but slower
 TRIM_PADDING_PIXELS = int(os.getenv('NOTES_CLEANER_TRIM_PADDING', '20'))  # Padding around content after trimming