improve osw.core.OSW.store_entity to also be aplicable for file pages

[LukasGold]

File pages can also be entities, but store entity does not account for suffixes like .png
At the moment this has to be worked around with page.edit() from wtsite.WtPage(title="sometitle_with.suffix")

[LukasGold]

Implemented. WikiFileController.put builds the suffixed title and delegates to OSW.store_entity, so file pages no longer need the raw WtPage.edit() workaround. See src/osw/controller/file/wiki.py and examples/file_upload_download.py for the current pattern.

Follow-up work on upload ordering and atomicity is tracked separately in #155 and #153.