Add professional project structure and documentation

New files:
- LICENSE (MIT) - Required legal file
- CHANGELOG.md - Version history following Keep a Changelog
- CONTRIBUTING.md - Contributor guidelines
- CODE_OF_CONDUCT.md - Community standards
- .github/ISSUE_TEMPLATE/ - Bug report and feature request forms
- .github/PULL_REQUEST_TEMPLATE.md - PR checklist
- src/stegasoo/py.typed - PEP 561 type hint marker
- examples/ - Usage examples (basic, file embedding, channel keys)

Updated:
- README.md - Added CI status badges

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

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

CHANGELOG.md

@@ -0,0 +1,96 @@
+# Changelog
+
+All notable changes to Stegasoo will be documented in this file.
+
+The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/),
+and this project adheres to [Semantic Versioning](https://semver.org).
+
+## [4.0.1] - 2025-01-02
+
+### Fixed
+- Fixed numpy binary incompatibility on Python 3.10 (jpegio/scipy)
+- Fixed BatchCredentials test failures with missing `reference_photo` parameter
+- Graceful handling when DCT dependencies have version mismatches
+
+### Changed
+- Applied `ruff` linter fixes across entire codebase (~400 issues)
+- Applied `black` formatter to all Python files
+- Modernized type hints: `Optional[X]` → `X | None`
+- Updated ruff config to use `[tool.ruff.lint]` section
+- Moved documentation files to repository root
+
+### Removed
+- Removed obsolete debug/diagnostic scripts
+- Cleaned up backup files and dev scripts
+
+## [4.0.0] - 2024-12-29
+
+### Added
+- Refreshed Web UI with modern, snazzy interface
+- Improved user experience across all pages
+
+### Changed
+- Major version bump for breaking API changes
+- Simplified passphrase handling (single passphrase instead of day-based)
+- Removed date_str parameter from encoding
+
+### Fixed
+- Various bug fixes for Web UI
+- CLI updates and improvements
+
+## [3.2.0] - 2024-12-28
+
+### Added
+- Big revamp of the encoding system
+- Home and about page improvements
+- UNDER_THE_HOOD.md documentation
+
+### Changed
+- Renamed `phrase` → `passphrase` in API
+- Updated Web UI styling
+
+## [3.0.2] - 2024-12-27
+
+### Added
+- Full experimental DCT steganography support
+- jpegio integration for better JPEG manipulation
+- DCT/LSB mode selector in Web UI
+
+## [3.0.0] - 2024-12-25
+
+### Added
+- DCT (Discrete Cosine Transform) steganography mode
+- Support for JPEG carriers without quality loss
+- Channel key feature for private messaging
+
+### Changed
+- Complete rewrite of steganography engine
+- New hybrid authentication system
+
+## [2.0.0] - 2024-12-20
+
+### Added
+- Web UI frontend
+- REST API (FastAPI)
+- Batch processing support
+- RSA key authentication option
+
+### Changed
+- Migrated to hybrid photo + passphrase + PIN authentication
+
+## [1.0.0] - 2024-12-15
+
+### Added
+- Initial release
+- LSB steganography
+- AES-256-GCM encryption
+- CLI interface
+- Basic PIN authentication
+
+[4.0.1]: https://github.com/adlee-was-taken/stegasoo/compare/v4.0.0...v4.0.1
+[4.0.0]: https://github.com/adlee-was-taken/stegasoo/compare/v3.2.0...v4.0.0
+[3.2.0]: https://github.com/adlee-was-taken/stegasoo/compare/v3.0.2...v3.2.0
+[3.0.2]: https://github.com/adlee-was-taken/stegasoo/compare/v3.0.0...v3.0.2
+[3.0.0]: https://github.com/adlee-was-taken/stegasoo/compare/v2.0.0...v3.0.0
+[2.0.0]: https://github.com/adlee-was-taken/stegasoo/compare/v1.0.0...v2.0.0
+[1.0.0]: https://github.com/adlee-was-taken/stegasoo/releases/tag/v1.0.0