feat(docker): add Docker deployment configuration

Add production-ready Docker setup with multi-stage Dockerfile, docker-compose
orchestration for API, PostgreSQL, Redis, and Nginx services. Includes
health checks, non-root user execution, and proper networking.

- Add multi-stage Dockerfile with gunicorn/gevent workers
- Add docker-compose.yml with api, db, redis, nginx services
- Add nginx reverse proxy configuration with security headers
- Update .env.example with Docker and production variables
- Add email provider configuration (Mailgun, SendGrid)
- Add requests dependency for HTTP client support
- Update documentation with Docker deployment guide
- Rebrand project name from Gatehouse to Secuird

Dockerfile

@@ -0,0 +1,68 @@
+# Multi-stage build for Gatehouse Auth API
+# Build stage
+FROM python:3.11-slim as builder
+
+# Install build dependencies
+RUN apt-get update && apt-get install -y --no-install-recommends \
+    build-essential \
+    libpq-dev \
+    curl \
+    && rm -rf /var/lib/apt/lists/*
+
+# Create virtual environment
+RUN python -m venv /opt/venv
+ENV PATH="/opt/venv/bin:$PATH"
+
+# Copy requirements files
+WORKDIR /app
+COPY requirements/base.txt requirements/base.txt
+COPY requirements/production.txt requirements/production.txt
+
+# Install dependencies
+RUN pip install --no-cache-dir --upgrade pip wheel && \
+    pip install --no-cache-dir -r requirements/production.txt
+
+# Production stage
+FROM python:3.11-slim
+
+# Install runtime dependencies
+RUN apt-get update && apt-get install -y --no-install-recommends \
+    libpq5 \
+    curl \
+    && rm -rf /var/lib/apt/lists/*
+
+# Create non-root user
+RUN groupadd --gid 1000 appgroup && \
+    useradd --uid 1000 --gid appgroup --shell /bin/bash --create-home appuser
+
+# Copy virtual environment from builder
+COPY --from=builder /opt/venv /opt/venv
+ENV PATH="/opt/venv/bin:$PATH"
+
+# Copy application code
+WORKDIR /app
+COPY --chown=appuser:appgroup . .
+
+# Create log and session directories
+RUN mkdir -p /app/logs /app/flask_session && chown -R appuser:appgroup /app/logs /app/flask_session
+
+# Switch to non-root user
+USER appuser
+
+# Expose port
+EXPOSE 5000
+
+# Health check
+HEALTHCHECK --interval=30s --timeout=10s --start-period=40s --retries=3 \
+    CMD curl -f http://localhost:5000/api/health || exit 1
+
+# Run gunicorn with gevent workers
+CMD ["gunicorn", "--bind", "0.0.0.0:5000", \
+     "--workers", "4", \
+     "--worker-class", "gevent", \
+     "--worker-connections", "1000", \
+     "--timeout", "120", \
+     "--access-logfile", "-", \
+     "--error-logfile", "-", \
+     "--log-level", "info", \
+     "wsgi:application"]