Revert backend Dockerfile to run as root

Non-root appuser can't write to the host-mounted SQLite data volume —
chown in the image layer doesn't carry over to runtime volume mounts.

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

btc-portfolio/backend/Dockerfile

@@ -5,12 +5,8 @@ WORKDIR /app
 COPY requirements.txt .
 RUN pip install --no-cache-dir -r requirements.txt
 
-RUN addgroup --system appgroup && adduser --system --ingroup appgroup appuser
-
 COPY . .
 
-RUN mkdir -p /app/data && chown -R appuser:appgroup /app/data
-
-USER appuser
+RUN mkdir -p /app/data
 
 CMD ["uvicorn", "app.main:app", "--host", "0.0.0.0", "--port", "8000"]