🚸(backend) sort user search results by proximity with the active user (#1802)

## Purpose
Allows a user to find more easily the other users they search, with the
following order of priority:
- users they already share documents with (more recent first)
- users that share the same full email domain
- ~~users that share the same partial email domain (last two parts)~~
- ~~other users~~

Edit: We need to ilter out other users in order to not reveal email
addresses from members of other organisations. It's still possible to
invite them by email.

Solves #1521

## Proposal
- [x] Add a new function in `core/utils.py`:
`users_sharing_documents_with()`
- [x] Use it as a key to sort the results of a basic user search
- [x] Filter user results to avoid reveal of users (and email addresses)
of other orgs or that have not been interacted with.
- [x] User research through "full" email address (contains the '@') is
left unaffected.

---------

Co-authored-by: Anthony LC <anthony.le-courric@mail.numerique.gouv.fr>

src/backend/core/tests/test_utils.py

@@ -3,9 +3,14 @@
 import base64
 import uuid
 
-import pycrdt
+from django.core.cache import cache
 
-from core import utils
+import pycrdt
+import pytest
+
+from core import factories, utils
+
+pytestmark = pytest.mark.django_db
 
 # This base64 string is an example of what is saved in the database.
 # This base64 is generated from the blocknote editor, it contains
@@ -100,3 +105,103 @@ def test_utils_get_ancestor_to_descendants_map_multiple_paths():
         "000100020005": {"000100020005"},
         "00010003": {"00010003"},
     }
+
+
+def test_utils_users_sharing_documents_with_cache_miss():
+    """Test cache miss: should query database and cache result."""
+    user1 = factories.UserFactory()
+    user2 = factories.UserFactory()
+    user3 = factories.UserFactory()
+    doc1 = factories.DocumentFactory()
+    doc2 = factories.DocumentFactory()
+
+    factories.UserDocumentAccessFactory(user=user1, document=doc1)
+    factories.UserDocumentAccessFactory(user=user2, document=doc1)
+    factories.UserDocumentAccessFactory(user=user3, document=doc2)
+
+    cache_key = utils.get_users_sharing_documents_with_cache_key(user1)
+    cache.delete(cache_key)
+
+    result = utils.users_sharing_documents_with(user1)
+
+    assert user2.id in result
+
+    cached_data = cache.get(cache_key)
+    assert cached_data == result
+
+
+def test_utils_users_sharing_documents_with_cache_hit():
+    """Test cache hit: should return cached data without querying database."""
+    user1 = factories.UserFactory()
+    user2 = factories.UserFactory()
+    doc1 = factories.DocumentFactory()
+
+    factories.UserDocumentAccessFactory(user=user1, document=doc1)
+    factories.UserDocumentAccessFactory(user=user2, document=doc1)
+
+    cache_key = utils.get_users_sharing_documents_with_cache_key(user1)
+
+    test_cached_data = {user2.id: "2025-02-10"}
+    cache.set(cache_key, test_cached_data, 86400)
+
+    result = utils.users_sharing_documents_with(user1)
+    assert result == test_cached_data
+
+
+def test_utils_users_sharing_documents_with_cache_invalidation_on_create():
+    """Test that cache is invalidated when a DocumentAccess is created."""
+    # Create test data
+    user1 = factories.UserFactory()
+    user2 = factories.UserFactory()
+    doc1 = factories.DocumentFactory()
+
+    # Pre-populate cache
+    cache_key = utils.get_users_sharing_documents_with_cache_key(user1)
+    cache.set(cache_key, {}, 86400)
+
+    # Verify cache exists
+    assert cache.get(cache_key) is not None
+
+    # Create new DocumentAccess
+    factories.UserDocumentAccessFactory(user=user2, document=doc1)
+
+    # Cache should still exist (only created for user2 who was added)
+    # But if we create access for user1 being shared with, cache should be cleared
+    cache.set(cache_key, {"test": "data"}, 86400)
+    factories.UserDocumentAccessFactory(user=user1, document=doc1)
+
+    # Cache for user1 should be invalidated (cleared)
+    assert cache.get(cache_key) is None
+
+
+def test_utils_users_sharing_documents_with_cache_invalidation_on_delete():
+    """Test that cache is invalidated when a DocumentAccess is deleted."""
+    user1 = factories.UserFactory()
+    user2 = factories.UserFactory()
+    doc1 = factories.DocumentFactory()
+
+    doc_access = factories.UserDocumentAccessFactory(user=user1, document=doc1)
+
+    cache_key = utils.get_users_sharing_documents_with_cache_key(user1)
+    cache.set(cache_key, {user2.id: "2025-02-10"}, 86400)
+
+    assert cache.get(cache_key) is not None
+
+    doc_access.delete()
+
+    assert cache.get(cache_key) is None
+
+
+def test_utils_users_sharing_documents_with_empty_result():
+    """Test when user is not sharing any documents."""
+    user1 = factories.UserFactory()
+
+    cache_key = utils.get_users_sharing_documents_with_cache_key(user1)
+    cache.delete(cache_key)
+
+    result = utils.users_sharing_documents_with(user1)
+
+    assert result == {}
+
+    cached_data = cache.get(cache_key)
+    assert cached_data == {}