Attachment handling improvements

During the team week we discussed various improvements to solve various issues that we have:

• Too big folders
• Virtual Sync issue Mac
• Memory limit
• Amount of mount points

• .attachments folder hiding all apps, read from oc_preferences in server: https://github.com/nextcloud/server/issues/46309
• Group by token https://github.com/nextcloud/spreed/issues/4340
  • Would allow https://github.com/nextcloud/spreed/issues/6469
• Group by Year/Month/
  • Still have 8k Mountpoints with this
  • OCC command to migrate files after upgrade to the new architecture
  • Would easily allow https://github.com/nextcloud/spreed/issues/7793
• Allow moderators to share a folder for guest / people without quota upload https://github.com/nextcloud/spreed/issues/3343
• User shares a folder one-time and then all their uploads go into this
  • Contradicts a bit with the Y/M/ separation from UX
  • Problem with everyone having "Shared Conversation name" or "Shared by user_id"
  • Would make https://github.com/nextcloud/spreed/issues/7462 impossible
• #15309
  • Sort into .attachments/{Talk}/{token}
  • Allow sharing as "other moderator" not only the one that started the recording
  • Not obvious when it's still "to share" or failed uploading etc.
  • Video, Summary and Transcribe are not linked/grouped together

Start by reviewing the listed attachment concerns and the linked issues, especially #15309, to understand the competing storage and sharing requirements. Done would require a decided attachment architecture that addresses the stated folder size, mount-point, grouping, sharing, upload-status, and linking concerns.