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>

frontends/api/main.py

@@ -1,10 +1,15 @@
 #!/usr/bin/env python3
 """
-Stegasoo REST API (v4.2.0)
+Stegasoo REST API (v4.2.1)
 
 FastAPI-based REST API for steganography operations.
 Supports both text messages and file embedding.
 
+CHANGES in v4.2.1:
+- API key authentication (X-API-Key header)
+- TLS support with self-signed certificates
+- /auth/* endpoints for key management
+
 CHANGES in v4.2.0:
 - Async encode/decode operations (run in thread pool)
 - Server can handle concurrent requests without blocking