chore: add docker infrastructure, gists feature, attachment system, and admin CLI tools

- Add .dockerignore, Dockerfile, and docker compose targets to Makefile for containerized deployment
- Implement gists router with CRUD operations, database schema, and polymorphic comment/vote reuse
- Create attachment upload system with thumbnail generation, MIME detection, and storage path management
- Add attachments_prune CLI command to clean orphaned attachment records and files
- Introduce rate limiting middleware with 60 requests per minute window
- Add custom 404 and 500 error handlers with SEO-optimized template responses
- Extend database initialization with gists and attachments indexes plus upload site settings defaults
- Update load_comments to include attachment mapping for comment resources
- Register gists and uploads routers in main application and update AGENTS.md documentation

Dockerfile

@@ -0,0 +1,22 @@
+FROM python:3.11-slim
+
+WORKDIR /app
+
+RUN apt-get update && apt-get install -y --no-install-recommends \
+    curl \
+    && rm -rf /var/lib/apt/lists/*
+
+COPY pyproject.toml .
+
+COPY devplacepy/ devplacepy/
+
+RUN pip install --no-cache-dir .
+
+RUN mkdir -p /app/data /app/devplacepy/static/uploads/attachments
+
+EXPOSE 10500
+
+HEALTHCHECK --interval=30s --timeout=10s --retries=3 --start-period=10s \
+    CMD curl -f http://localhost:10500/ || exit 1
+
+CMD ["uvicorn", "devplacepy.main:app", "--host", "0.0.0.0", "--port", "10500", "--workers", "4", "--backlog", "8192"]