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>

.gitignore

@@ -9,3 +9,4 @@ backups/
 scan/
 dist-source/
 .worktrees/
+scripts/azurite/