Add Flask shopping app with Nomad deployment

Flask app serving the Citti shopping list with SQLite persistence,
per-category item addition, dynamic custom categories, photo upload,
and a post-trip price analysis page.

- Port 9756, DATA_DIR env var for persistent volume in Nomad
- Gitea Actions pipeline builds Docker image and deploys via Nomad API
- Makefile targets: run, install, build, push

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

Dockerfile

@@ -0,0 +1,19 @@
+FROM python:3.12-slim
+
+WORKDIR /app
+
+COPY requirements.txt .
+RUN pip install --no-cache-dir -r requirements.txt
+
+COPY app.py .
+COPY templates/ templates/
+COPY static/ static/
+
+ENV DATA_DIR=/data
+VOLUME ["/data"]
+EXPOSE 9756
+
+RUN useradd -m appuser && chown -R appuser /app
+USER appuser
+
+CMD ["python", "app.py"]