test: migrate backend suites to pytest with module coverage gate

Split backend tests into unit and integration suites, add isolated fixtures and coverage enforcement, and update CI and test documentation. Cover current authentication, content, notification, payment, media, and admin flows; fix behaviors revealed by the new tests. Validate both suites and the per-module gate against local PostgreSQL.

apps/users/tests/unit/test_identity.py

@@ -0,0 +1,42 @@
+import pytest
+
+from apps.users.email_identity import (
+    is_placeholder_email,
+    is_valid_mobile_number,
+    mask_mobile,
+    normalize_email_identity,
+    normalize_mobile_number,
+)
+
+
+pytestmark = pytest.mark.unit
+
+
+@pytest.mark.parametrize(
+    ("raw", "expected"),
+    [(None, None), ("  ", None), (" Alice@EXAMPLE.COM ", "alice@example.com")],
+)
+def test_email_normalization(raw, expected):
+    assert normalize_email_identity(raw) == expected
+
+
+@pytest.mark.parametrize(
+    ("raw", "normalized", "valid"),
+    [
+        (None, None, False),
+        ("+98 (0919) 000-0001", "9809190000001", False),
+        ("0919-000-0001", "09190000001", True),
+        ("123", "123", False),
+    ],
+)
+def test_mobile_normalization_and_validation(raw, normalized, valid):
+    assert normalize_mobile_number(raw) == normalized
+    assert is_valid_mobile_number(raw) is valid
+
+
+def test_masking_and_placeholder_identity():
+    assert mask_mobile(None) is None
+    assert mask_mobile("1234") == "1234"
+    assert mask_mobile("09190000001") == "09*****0001"
+    assert is_placeholder_email(" LEGACY@NOEMAIL.LOCAL ")
+    assert not is_placeholder_email("person@example.com")