feat: Phase 6 — Docker + docker-compose + backup/restore CLI

- Dockerfile: multi-stage build (nim:alpine -> alpine), env-configurable
- docker-compose.yml: single node with volume + healthcheck
- backup.nim: backup/restore tar.gz snapshots, list/cleanup
- CLI: baradadb backup|restore|list --data-dir=DIR
- All 216 tests pass

Dockerfile

@@ -0,0 +1,18 @@
+FROM nim:2.2-alpine AS builder
+WORKDIR /build
+COPY baradadb.nimble .
+COPY src/ src/
+RUN nimble install -d -y || true
+RUN nim c -d:release --opt:speed -o:baradadb src/baradadb.nim
+
+FROM alpine:latest
+WORKDIR /app
+COPY --from=builder /build/baradadb .
+RUN mkdir -p /data
+ENV BARADB_PORT=9000
+ENV BARADB_DATA_DIR=/data
+ENV BARADB_HTTP_PORT=8080
+ENV BARADB_WS_PORT=8081
+EXPOSE 9000 8080 8081
+VOLUME ["/data"]
+CMD ["./baradadb"]