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'Perform high volume activity (High-volume editing)': I'm not sure about that. What is low volume? What if 10 users upload images in the very same minute?
'Interact with pages (Edit existing pages; Create, edit, and move pages)': uploading new pictures requires to create pages, updating pictures requires to edit pages and renaming them should be covered as well)
'Interact with media (Upload new files; Upload, replace, and move files)': ditto
'Perform administrative actions (View deleted files and pages; Delete pages, revisions, and log entries)': I'm not sure about viewing deleted stuff, but at least deleting mistakes should be covered.
OAuth is an authentication method; actions will be handled as if performed by the user, not the app. So throttling will count against the individual user's limits and the highvolume grant means a single user can make more then 10 actions a minute. Grants can only give rights the user already has, so that's not a huge deal but still better not to give it unless actually needed (an attacker hijacking the app can deal more damage if there are no rate limits).
Deletion is subject to various policy restrictions; few users have the right to use it and even they are only supposed to use it some specific ways. If the OsmHydrant workflow involves image deletion, I'd suggest discussing it with the Commons community first. --Tgr (WMF) (talk) 23:09, 20 January 2019 (UTC)
All the edits from the app OsmHydrant I plan to do with the user OsmHydrant (although a small amount of users might prefer to use their own Wikimedia user). This means there might be more than 10 actions a minute. Within the OsmHydrant app, users are logged in with their OpenStreetMap user. Image uploading and deletion should just work, without another login. Hence using the existing Wikimedia user OsmHydrant eases the experience, while I'll still be able to know which OpenStreetMap user uploaded/deleted which pictures. How should I discuss my use case with the commons community? Contact special admins or post into a mailing list?
If you want to use a dedicated user, you should set up an owner-only consumer instead (which does not require approval), and ask for bot permissions (that includes high-volume editing). (And maybe sysop permissions for deletion, although I don't think there's any chance of being granted that. Your app should probably tag images for deletion instead.)
For discussion the village pump is the best place, although it's not very user-friendly if you are completely unfamiliar with Wikimedia communities. There's also a mailing list. --Tgr (WMF) (talk) 04:11, 21 January 2019 (UTC)