### Summary
- Make `BLOB_STORAGE_SIZE_THRESHOLD` configurable through an environment
variable.
- Preserve the existing 20 MiB default.
- Add tests for the default and configured values.
### Why
Blob storage, Seafile, and WebDAV connectors currently use a hardcoded
20 MiB limit. Self-hosted users
cannot raise this limit without modifying the source code inside the
container.
### Testing
- `test/unit_test/data_source/test_config.py`: 2 passed
- `ruff check common/data_source/config.py
test/unit_test/data_source/test_config.py`
Fixes#16634