Bump version to 4.2.1

Release highlights:
- API key authentication (X-API-Key header)
- TLS with self-signed certificates
- CLI tools: compress, rotate, convert
- jpegtran lossless JPEG rotation
- AUR packages: stegasoo-cli-git, stegasoo-api-git
- Bug fixes: DCT rotation, jpegtran -trim, CLI output format

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

aur-cli/PKGBUILD

@@ -29,7 +29,7 @@ sha256sums=('SKIP')
 pkgver() {
     cd "$pkgname"
     git describe --long --tags 2>/dev/null | sed 's/^v//;s/\([^-]*-g\)/r\1/;s/-/./g' || \
-    printf "%s.r%s.g%s" "4.2.0" "$(git rev-list --count HEAD)" "$(git rev-parse --short HEAD)"
+    printf "%s.r%s.g%s" "4.2.1" "$(git rev-list --count HEAD)" "$(git rev-parse --short HEAD)"
 }
 
 build() {