Move express_search app under backend/express_search/

[pashiav]

Fixes #6726

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

Summary of Changes:

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

This is the 7th 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 Reorganize notifications app under backend/notifications/ #6707.

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.

See the Simple Search documentation.

• General Simple Search testing (additional gen testing in addition to the ones below)
  • Test the functionality and results when using the asterisk (*) as a wild card.
  • Open record from list
  • Use “Browse in Forms” button
  • Perform Some Record Merging In Simple Search Results

[bhumikaguptaa]

• General Simple Search testing
  • Test the functionality and results when using the asterisk (*) as a wild card.
  • Open record from list
  • Use “Browse in Forms” button
  • Perform Some Record Merging In Simple Search Results

Works as expected!

[emenslin]

• General Simple Search testing (additional gen testing in addition to the ones below)
  • Test the functionality and results when using the asterisk (*) as a wild card.
  • Open record from list
  • Use “Browse in Forms” button
  • Perform Some Record Merging In Simple Search Results