docs: add comprehensive security and authentication documentation

Add new documentation sections covering security best practices and authentication system architecture. Update Sphinx configuration and dependencies to support documentation improvements.

Changes include:
- New security.rst with SSH key management, network security, secrets management
- New authentication.rst documenting pluggable auth system and provider setup
- Updated Sphinx config to use Alabaster theme and add sphinx-tabs extension
- Added docs extra dependencies in pyproject.toml for documentation builds
- Updated example configs to use Ed25519 instead of deprecated DSA keys
- Added .python-version file for consistent Python version management
- Added CLAUDE.md project instructions for AI-assisted development
- Minor Dockerfile cleanup removing commented pip install line

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

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

pyproject.toml

@@ -25,6 +25,15 @@ auth-login = ["flask-login>=0.6.0", "bcrypt>=4.0.0"]
 auth-oauth = ["authlib>=1.3.0", "requests>=2.32.0"]
 # Install all auth providers
 auth-all = ["bcrypt>=4.0.0", "flask-login>=0.6.0", "authlib>=1.3.0", "requests>=2.32.0"]
+# Documentation dependencies
+docs = [
+    "docutils",
+    "sphinx>=7.0.0,<8.0.0",
+    "sphinx_rtd_theme",
+    "sphinxjp.themes.revealjs",
+    "sphinx-intl",
+    "sphinx-tabs",
+]
 
 [tool.black]
 line-length = 140