Fix /data permissions: create directory owned by app user

Docker mounts named volumes as root by default. Without pre-creating /data
in the image with correct ownership, the app user cannot write tasks.db,
causing a 502 on any route that touches TaskStore.

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

Dockerfile

@@ -20,6 +20,7 @@ RUN sbt "set test in assembly := false" assembly
 FROM eclipse-temurin:25-jre-alpine
 
 RUN addgroup -S app && adduser -S app -G app
+RUN mkdir -p /data && chown app:app /data
 WORKDIR /app
 
 COPY --from=build --chown=app:app /app/target/scala-3.7.1/demoapp.jar ./demoapp.jar