Tasks for preserving data integrity

[PVince81]

Primary goal is to get rid of recurring issues like "user lost shares" or "user lost files".
Secondary goal is to avoid having leftover/orphaned entries (these can be cleaned up by repair steps, but still ugly to have in the first place)

Since the code can never be bug-free, we should strive and add more safeties in the code and data structures.

This ticket is a list of tasks that can help improve on this:

• use foreign keys / cascade delete in database: Introduce foreign keys / cascading delete #13143
• fix the storage table / use proper database design for storages / link storages with users directly: [ext storage] Rely on numeric_id, store config in DB, improve config format #11261 (comment)
• use DB transactions ?
• stable file ids: File ids must stay stable #13310
• stable file ids for remote changes on external storage: externally renaming a file/folder in a files_external storage breaks the contained shares  #11533
• checksums for files: Compute and store file checksums on the fly / file integrity #11811 Checksums to Verify Up- and Download #12417
• defensive programming: Add more checks for expected values / defensive programming #13885
• data must be able to survive PHP timeouts and/or be rolled back when it happens: async file operations Async file operations #24509

@DeepDiver1975 @icewind1991 @schiesbn

[PVince81]

• properly roll back database operations
  • roll back file operations, potential idea here Update file cache before doing file operation instead of after, rollback possibility #24665