Local source episodes thumbnails keep generating with each refrash

### Steps to reproduce

Refrash inside "local source" title. That contain video files.

### Expected behavior

Thumbnails shouldn't generate (for each episodes) in each refrash. It should only generate for the first time and after only when a new video file are added. And without waiting for generating thumbnails, new added episodes should load right away with blank thumbnail.

### Actual behavior

The thumbnail for each episodes keep generating each time I refrash. And When I look from file manager. It replacing/overwriting previously generated thumbnails one by one. And new added episodes not coming in app without generating all episodes thumbnails from starts.

### Crash logs

_No response_

### Aniyomi version

0.18.1.0

### Android version

13

### Device

Symphony innova 30

### Other details

_No response_

### Acknowledgements

- [x] I have searched the existing issues and this is a new ticket, **NOT** a duplicate or related to another open or closed issue.
- [x] I have written a short but informative title.
- [x] If this is an issue with an extension, I should be opening an issue in the extension's repository.
- [x] I have gone through the [FAQ](https://aniyomi.org/docs/faq/general) and [troubleshooting guide](https://aniyomi.org/docs/guides/troubleshooting/).
- [x] I have updated the app to version **[0.18.1.0](https://github.com/aniyomiorg/aniyomi/releases/latest)**.
- [x] I have filled out all of the requested information in this form, including specific version numbers.
- [x] I understand that **Aniyomi does not have or fix any extensions**, and I **will not receive help** for any issues related to sources or extensions.