chore: production docker + compose for LAN deployment

Dockerfile

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+# syntax=docker/dockerfile:1
+
+FROM python:3.12-slim AS runtime
+
+ENV PYTHONDONTWRITEBYTECODE=1 \
+    PYTHONUNBUFFERED=1
+
+WORKDIR /app
+
+# System deps (kept minimal). We install curl for a simple container healthcheck.
+RUN apt-get update \
+    && apt-get install -y --no-install-recommends curl \
+    && rm -rf /var/lib/apt/lists/*
+
+# Create non-root user
+RUN useradd -r -u 10001 -g root syncplayer
+
+COPY requirements.txt ./
+RUN pip install --no-cache-dir -r requirements.txt
+
+# Copy app code
+COPY . .
+
+# Default persistent directories (can be mounted).
+ENV DATA_DIR=/data \
+    MEDIA_DIR=/data/media \
+    LOG_DIR=/data/logs \
+    HOST=0.0.0.0 \
+    PORT=5000 \
+    ASYNC_MODE=eventlet
+
+RUN mkdir -p /data/media /data/logs \
+    && chown -R syncplayer:root /data
+
+USER syncplayer
+
+EXPOSE 5000
+
+# Basic HTTP healthcheck (admin redirect)
+HEALTHCHECK --interval=15s --timeout=3s --start-period=10s --retries=3 \
+  CMD curl -fsS http://127.0.0.1:${PORT}/ >/dev/null || exit 1
+
+# Gunicorn + eventlet is a solid choice for Flask-SocketIO.
+# We intentionally run a single worker to avoid starting multiple UDP listeners.
+CMD ["gunicorn", "-k", "eventlet", "-w", "1", "-b", "0.0.0.0:5000", "wsgi:app"]