Move notifications app under backend/notifications/

[pashiav]

Fixes #6707

This PR reorganizes the project structure by moving the notifications app into a new backend/notifications/ directory under specifyweb/.

Summary of Changes:

• Moved notifications to specifyweb/backend/notifications/
• Updated all import paths to reflect the new location
• Updated SPECIFY_APPS in settings.py to include specifyweb.backend.notifications

This is the 6th PR in a broader backend reorganization. No frontend behavior or functionality was changed.

Notes:

• This PR requires a rebuild for the changes to take effect properly.
• This PR is built on Move raven_placeholder app under backend/raven_placeholder/ #6706.

Checklist

• Self-review the PR after opening it to make sure the changes look good and
  self-explanatory (or properly documented)
• Add relevant issue to release milestone
• Add pr to documentation list
• Add automated tests
• Add a reverse migration if a migration is present in the PR

Testing instructions

Please cross check the following functionalities with main.

• General Notifications testing

  Note that notifications may take a moment to pop up. Verify that you can:

  Record Merging (Documentation):

  • Query on the Agent table
  • Select 2 records -> Click on Merge Records
  • Merge those records
  • Receive a notification that the merge process has started
  • Receive a notification that the merge process has succeeded

  Queries (Note: Exported CSV files from queries do not always notify the user #4566):

  • Run a Locality query (e.g. https://sp7demofish20250210-issue-6707.test.specifysystems.org/specify/query/61/ - Note: I added a Timestamp Created filter to select less records to make the export quicker.)
  
  • Click Create a CSV or Create KML
  • Receive a notification containing the CSV/KML when completed

  Other:

  • Clicking Clear All removes all notifications.
    - Note: This button shows up when there is more than 1 notification

[bhumikaguptaa]

[X] General Notifications testing
- [X] Receive a notification that the merge process has started
- [X] Receive a notification that the merge process has succeeded
- [X] Receive a notification containing the CSV/KML when completed
- [X] Clicking Clear All removes all notifications.

Works as expected!

[emenslin]

• General Notifications testing
  • Receive a notification that the merge process has started
  • Receive a notification that the merge process has succeeded
  • Receive a notification containing the CSV/KML when completed
  • Clicking Clear All removes all notifications.

Looks good