feat: implement comprehensive environment variable credential support

- Add environment variable overrides for sensitive credentials in both Go and Rust implementations
- Support MAIL2COUCH_COUCHDB_USER and MAIL2COUCH_COUCHDB_PASSWORD for CouchDB credentials
- Support MAIL2COUCH_IMAP_<NAME>_USER and MAIL2COUCH_IMAP_<NAME>_PASSWORD for IMAP credentials
- Implement automatic name normalization for mail source names to environment variable format
- Add runtime display of active environment variable overrides
- Enhance --help output in both implementations with comprehensive environment variable documentation
- Add detailed environment variable section to README with usage examples and security benefits
- Create comprehensive ENVIRONMENT_VARIABLES.md reference guide with SystemD, Docker, and CI/CD examples
- Update all documentation indices and cross-references
- Include security best practices and troubleshooting guidance
- Maintain full backward compatibility with existing configuration files

This enhancement addresses the high-priority security requirement to eliminate plaintext
passwords from configuration files while providing production-ready credential management
for both development and deployment scenarios.

🤖 Generated with [Claude Code](https://claude.ai/code)

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

docs/README.md

@@ -11,6 +11,7 @@ This directory contains comprehensive documentation for the mail2couch project,
 - **[TODO.md](TODO.md)** - Development roadmap and outstanding tasks
 
 ### Configuration & Setup
+- **[ENVIRONMENT_VARIABLES.md](ENVIRONMENT_VARIABLES.md)** - Complete guide to environment variable credential overrides
 - **[FOLDER_PATTERNS.md](FOLDER_PATTERNS.md)** - Guide to folder filtering patterns and wildcards
 - **[test-config-comparison.md](test-config-comparison.md)** - Configuration examples and testing scenarios