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>

Dockerfile

@@ -13,7 +13,6 @@ ENV UV_PYTHON_DOWNLOADS=never
 RUN apt-get update && apt-get upgrade -y \
    && apt-get install --no-install-recommends -y rsync ssh cron \
    && rm -rf /var/lib/apt/lists/* \
-   #&& /usr/local/bin/python3.13 -m pip install --no-cache-dir -r requirements.txt \
    && uv sync --locked --no-dev --no-install-project \
    && rm -f /bin/uv /bin/uvx \
    && mkdir -p /var/spool/cron/crontabs \