Move barvis app under backend/barvis/

[pashiav]

Fixes #6701

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

Summary of Changes:

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

This is the 4th 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 attachment_gw app under backend/attachment_gw/ #6626.

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 functionality with main.

• Go to User Tools -> Preferences -> Home Page (url: .../specify/user-preferences/#welcomePage)
• Change Content to Taxon Tiles
• Go to the home page
• Verify that you can see the Taxon Tiles and they display as expected

[bhumikaguptaa]

[X] Verify that you can see the Taxon Tiles and they display as expected

Looks good and is working as expected!

[emenslin]

• Verify that you can see the Taxon Tiles and they display as expected

Looks good!