feat: initialize cart app with blueprints, templates, and CI

Extract cart, order, and orders blueprints with their service layer,
templates, Dockerfile (APP_MODULE=app:app, IMAGE=cart), entrypoint,
and Gitea CI workflow from the coop monolith.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Dockerfile

@@ -0,0 +1,33 @@
+# syntax=docker/dockerfile:1
+
+# ---------- Python application ----------
+FROM python:3.11-slim AS base
+
+ENV PYTHONDONTWRITEBYTECODE=1 \
+    PYTHONUNBUFFERED=1 \
+    PIP_NO_CACHE_DIR=1 \
+    APP_PORT=8000 \
+    APP_MODULE=app:app
+
+WORKDIR /app
+
+# Install system deps + psql client
+RUN apt-get update && apt-get install -y --no-install-recommends \
+      ca-certificates \
+      postgresql-client \
+    && rm -rf /var/lib/apt/lists/*
+
+COPY requirements.txt ./requirements.txt
+RUN pip install -r requirements.txt
+
+COPY . .
+
+# ---------- Runtime setup ----------
+COPY entrypoint.sh /usr/local/bin/entrypoint.sh
+RUN chmod +x /usr/local/bin/entrypoint.sh
+
+RUN useradd -m -u 10001 appuser && chown -R appuser:appuser /app
+USER appuser
+
+EXPOSE ${APP_PORT}
+ENTRYPOINT ["/usr/local/bin/entrypoint.sh"]