feat: add backup.retain_days config and local docker-compose for integration tests

- Add [backup] retain_days to config (default 1); --days becomes optional
- Add scripts/docker-compose.yml (azurite) for local integration testing
- Integration tests now use assets/local.toml and localhost endpoints
- Update .gitignore and CLAUDE.md accordingly

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

scripts/docker-compose.yml

@@ -0,0 +1,9 @@
+services:
+  azure:
+    image: mcr.microsoft.com/azure-storage/azurite
+    ports:
+      - 10000:10000
+      - 10001:10001
+      - 10002:10002
+    volumes:
+      - ./azurite:/data