Add multi-select metadata extraction

[paullizer]

Summary

• Add batch metadata extraction routes for personal, group, and public workspace documents.
• Add multi-select Extract Metadata actions to personal, group, and public workspace document bulk action bars.
• Preserve title updates by routing batch extraction through the shared metadata extraction background path and adding functional coverage.

Fixes #1134

Validation

• python functional_tests\test_multiselect_metadata_extraction.py
• python -m py_compile application\single_app\route_backend_documents.py application\single_app\route_backend_group_documents.py application\single_app\route_backend_public_documents.py functional_tests\test_multiselect_metadata_extraction.py
• node --check application\single_app\static\js\workspace\workspace-documents.js
• node --check application\single_app\static\js\public\public_workspace.js
• python functional_tests\route_tests\test_route_blueprint_policy_inventory.py
• python functional_tests\route_tests\test_route_unauthenticated_policy_contract.py
• python functional_tests\route_tests\test_route_policy_test_coverage.py
• git -c core.whitespace=blank-at-eol,blank-at-eof,space-before-tab,cr-at-eol diff --check