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.

core/tests/unit/test_formatting.py

@@ -0,0 +1,24 @@
+from datetime import datetime, timezone
+
+import pytest
+
+from core.choices import MajorChoices
+from core.templatetags.jalali import fa_digits, jdate
+
+
+pytestmark = pytest.mark.unit
+
+
+def test_persian_digits_translate_numbers_without_changing_text():
+    assert fa_digits("Year 1404/09") == "Year ۱۴۰۴/۰۹"
+    assert fa_digits(12) == "۱۲"
+
+
+def test_jalali_date_handles_missing_and_aware_values():
+    assert jdate(None) == ""
+    assert jdate(datetime(2025, 3, 21, tzinfo=timezone.utc), "%Y/%m/%d") == "1404/01/01"
+
+
+def test_major_choice_public_values_are_stable():
+    assert MajorChoices.CE.value == "CE"
+    assert dict(MajorChoices.choices)["CE"]