Issue title: Manually scanning app data
 Issue body: Since this app exists I always had the problem that after running `occ preview:pre-generate` my photos still seemed to load very slowly (via the gallery app and the Android app). During this time the CPU load on the server was also quite high, as if the previews were created for the first time.

If i opened the folder in the browser via the files app all previews loaded fast and CPU load was low -> pregenerated previews were used I guess.
I always assumed that this behaviour was normal and my server simply didn't have enough resources to show the previews quickly.

However, now I found a workaround:

I run `occ file:scan-app-data` after `occ preview:pre-generate` everything is fast and CPU load stays relatively low. My Nextcloud and the Android app are now finally usable to watch large photo collections (>200GB) :tada: :smile: 

Therefore I have updated my cron job to:
```
0    6    *    *   * php -f /var/www/html/occ preview:pre-generate
0    7    *    *   * php -f /var/www/html/occ file:scan-app-data
```

Is this behaviour excpected? Maybe someone can reproduce this too?
Is it a good idead to run `occ file:scan-app-data` regularly or are the any downsides?
 Comments: 
 Comment 0: Is there any update on this?
I tried to run `occ file:scan-app-data` and my database size increased by 20%. Even after running `occ file:cleanup` it returned that there are no orphaned files.

I never added anything manually to my Nextcloud so surely all those new entries must come from this app.
{'source': 'GithubIssues', 'repo_name_with_owner': 'rullzer/previewgenerator', 'issue_id': 440368788, 'repo_stars': 221.0}