Update docs and comments for jpeglib migration (v4.2.0)

- Replace jpegio references with jpeglib in comments/docstrings
- Update sanitize-for-image.sh to use system Python 3.11+ (no pyenv)
- Update rpi/patches/README.md for jpeglib world
- Add AUR build artifacts to .gitignore

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

frontends/web/app.py

@@ -31,7 +31,7 @@ KEY PATTERNS
 ============
 
 1. SUBPROCESS ISOLATION
-   Stegasoo's DCT mode uses scipy/jpegio which can crash on malformed input.
+   Stegasoo's DCT mode uses scipy/jpeglib which can crash on malformed input.
    We run encode/decode in subprocesses so crashes don't take down the server:
 
        subprocess_stego = SubprocessStego(timeout=180)
@@ -213,7 +213,7 @@ except ImportError:
 #
 # This is a critical reliability pattern. Here's the problem:
 #
-# scipy's DCT and jpegio can crash (segfault) on:
+# scipy's DCT and jpeglib can crash (segfault) on:
 # - Malformed JPEG files
 # - Very large images that exhaust memory
 # - Certain edge cases in coefficient manipulation