Add SQLite persistence via Docker named volume

Tasks are now stored in SQLite (DB_PATH env var, defaults to ./tasks.db).
Pre-seeding runs only when the table is empty, so upgrades preserve data.
This is the v1.0.0 baseline for the persistence demo.

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

src/main/resources/site.conf

@@ -1,8 +1,8 @@
 app {
   name = "DemoCust Tasks"
-  version = "2.0.0"
+  version = "1.0.0"
   theme {
-    background = "#1D4ED8"
+    background = "#0F766E"
     font = "DM Sans"
     fontSize = "16px"
   }