v1.0.3: feat: Add configurable memory limit for exports

- Introduce a new `memoryLimit` configuration option, allowing projects to explicitly set PHP's `memory_limit` for export generation. This is important because exports can run synchronously within HTTP requests, and the library leaves the decision for large limits to the project.
- Improve export processing robustness:
    - The `processedAt` timestamp is now marked only after the export file has been successfully generated AND delivered (e.g., via email). This ensures an export isn't considered complete if the user hasn't received it, and enables proper retries if delivery fails.
    - When purging old export files, the entity manager is flushed after each file removal. This makes the purge operation more resilient to crashes, as the `filesPurgedAt` status is immediately persisted for each processed export.
- Fix `ExportSection` logic for empty data sources: Empty arrays for aggregated sections (where `entityClass` is `null`) are now correctly identified as raw rows. Previously, this would incorrectly be treated as an empty ID list, causing errors for sections like summaries without sales.