fix: unify DATA_DIR default to /var/lib/webhooker for all environments

Remove devDataDir() XDG-based logic. Both dev and prod now default
DATA_DIR to /var/lib/webhooker. Update Dockerfile and README to match.

Dockerfile

@@ -57,10 +57,10 @@ WORKDIR /app
 COPY --from=builder /build/bin/webhooker .
 
 # Create data directory for all SQLite databases (main app DB +
-# per-webhook event DBs). DATA_DIR defaults to /data in production.
-RUN mkdir -p /data
+# per-webhook event DBs). DATA_DIR defaults to /var/lib/webhooker.
+RUN mkdir -p /var/lib/webhooker
 
-RUN chown -R webhooker:webhooker /app /data
+RUN chown -R webhooker:webhooker /app /var/lib/webhooker
 
 USER webhooker