Refactor: Unified Auto Upload Architecture

[PhilippThaler]

Built upon #56 and based on #124
So there aren't that many changes if #56 is merged first

Summary

This PR is mostly about refactoring the background workers to use a unified upload architecture. Instead of having "Picture" and "Video" logic hardcoded everywhere, the background sync is now entirely driven by the configuration itself.
This gives us a much cleaner foundation so we can eventually support custom folders and other features for auto uploading in the future.
While creating this, I also fixed a few edge cases, I ran into where settings would overwrite each other, or where files would end up in the wrong folder if you changed your settings while waiting for Wi-Fi.

What changed

• Unified the upload logic: The workers no longer care if a file is a picture or a video. They just look up the configuration and do what it says.
• Moved the subfolder logic: I moved the buildUploadPath function from Feature: Add Subfolder Strategy for Automatic Uploads #56 into a shared helper (AutoUploadPathBuilder) so both the scanner and the actual upload worker can use it.
• Calculate paths at the very last second: Before, the app locked in the upload path as soon as it found a new file. Now, it calculates the final path right before it hits the network. This means if a file is waiting for Wi-Fi and you change your destination folder in the settings, it will actually go to the new folder.
• Fixed settings getting overwritten: The worker now fetches the latest settings right before saving its sync timestamp. This stops it from accidentally restoring old settings if you changed them while the worker was running.
• Fixed picture/video mixups: Because we moved to a unified config system, I added a strict check on the config name. This stops the app from mixing up picture and video settings if they are both watching the exact same local folder (like /DCIM/Camera).

How to test

1. Normal uploads still work (Regression)
   • Turn on auto-uploads with subfolders, take a pic/video, and make sure it still works exactly like it did in Feature: Add Subfolder Strategy for Automatic Uploads #56. The unified logic shouldn't break anything.
2. Changing settings mid-upload
   • Turn on auto-uploads (Wi-Fi only) and turn off your Wi-Fi.
   • Take a picture/video so it gets queued up.
   • Go into the app settings and change the destination folder.
   • Turn Wi-Fi back on.
   • Verify: It should upload to the new folder, and the Uploads tab should update to show the new folder right as it starts.
3. Picture/Video mixup fix
   • Set up both Picture and Video auto-uploads for the exact same local folder (e.g., /DCIM/Camera) but with different target folders on the server.
   • Trigger an auto-upload.
   • Verify: The picture and video configs should stay completely separate and not overwrite each other in the database anymore.

So this