feat: add production container support with health check endpoint

Implements Phase 5 containerization specification:
- Add /health endpoint for container monitoring
- Create multi-stage Containerfile (Podman/Docker compatible)
- Add compose.yaml for orchestration
- Add Caddyfile.example for reverse proxy (auto-HTTPS)
- Add nginx.conf.example as alternative
- Update .env.example with container and RSS feed variables
- Add gunicorn WSGI server to requirements.txt

Container features:
- Multi-stage build for smaller image size
- Non-root user (starpunk:1000)
- Health check with database connectivity test
- Volume mount for data persistence
- Resource limits and logging configuration
- Security headers and HTTPS configuration examples

Health check endpoint:
- Tests database connectivity
- Verifies filesystem access
- Returns JSON with status, version, and environment

Following Phase 5 design in docs/designs/phase-5-rss-and-container.md

Containerfile

@@ -0,0 +1,83 @@
+# syntax=docker/dockerfile:1
+
+# Multi-stage build for StarPunk production container
+# Podman and Docker compatible
+
+# ============================================================================
+# Build Stage - Install dependencies in virtual environment
+# ============================================================================
+FROM python:3.11-slim AS builder
+
+# Install uv for fast dependency installation
+COPY --from=ghcr.io/astral-sh/uv:latest /uv /usr/local/bin/uv
+
+WORKDIR /build
+
+# Copy dependency files
+COPY requirements.txt .
+
+# Create virtual environment and install dependencies
+# Using uv for fast, reproducible installs
+RUN uv venv /opt/venv && \
+    . /opt/venv/bin/activate && \
+    uv pip install --no-cache -r requirements.txt
+
+# ============================================================================
+# Runtime Stage - Minimal production image
+# ============================================================================
+FROM python:3.11-slim
+
+# Create non-root user for security
+# UID/GID 1000 is standard for first user on most systems
+RUN useradd --uid 1000 --create-home --shell /bin/bash starpunk && \
+    mkdir -p /app /data/notes && \
+    chown -R starpunk:starpunk /app /data
+
+# Copy virtual environment from builder stage
+COPY --from=builder /opt/venv /opt/venv
+
+# Set environment variables
+ENV PATH="/opt/venv/bin:$PATH" \
+    PYTHONUNBUFFERED=1 \
+    PYTHONDONTWRITEBYTECODE=1 \
+    FLASK_APP=app.py \
+    DATA_PATH=/data \
+    NOTES_PATH=/data/notes \
+    DATABASE_PATH=/data/starpunk.db
+
+WORKDIR /app
+
+# Copy application code
+COPY --chown=starpunk:starpunk . .
+
+# Switch to non-root user
+USER starpunk
+
+# Expose application port
+EXPOSE 8000
+
+# Health check
+# Uses httpx (already in requirements) to verify app is responding
+HEALTHCHECK --interval=30s --timeout=3s --start-period=10s --retries=3 \
+    CMD python3 -c "import httpx; httpx.get('http://localhost:8000/health', timeout=2.0)" || exit 1
+
+# Run gunicorn WSGI server
+# - 4 workers for concurrency (adjust based on CPU cores)
+# - Sync worker class (simple, reliable)
+# - Worker tmp dir in /dev/shm (shared memory, faster)
+# - Worker recycling to prevent memory leaks
+# - 30s timeout for slow requests
+# - Log to stdout/stderr for container log collection
+CMD ["gunicorn", \
+     "--bind", "0.0.0.0:8000", \
+     "--workers", "4", \
+     "--worker-class", "sync", \
+     "--worker-tmp-dir", "/dev/shm", \
+     "--max-requests", "1000", \
+     "--max-requests-jitter", "50", \
+     "--timeout", "30", \
+     "--graceful-timeout", "30", \
+     "--access-logfile", "-", \
+     "--error-logfile", "-", \
+     "--log-level", "info", \
+     "app:app"]