WIP checkpoint: Enforce 18+ age gate at account registration

backend/pytest.ini

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+[pytest]
+asyncio_mode = auto
+testpaths = tests

backend/tests/test_auth_age_gate.py

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+"""Tests for the 18+ age gate on user registration."""
+
+from datetime import date, timedelta, datetime
+from unittest.mock import patch
+
+import pytest
+
+from app.routers.auth import calculate_age
+
+
+class TestCalculateAge:
+    """Unit tests for the calculate_age helper."""
+
+    def test_exactly_18(self):
+        """A person born exactly 18 years ago today is 18."""
+        today = datetime.now().date()
+        dob = today.replace(year=today.year - 18)
+        assert calculate_age(dob) == 18
+
+    def test_just_turned_18(self):
+        """A person who turned 18 yesterday is 18."""
+        today = datetime.now().date()
+        dob = today.replace(year=today.year - 18) - timedelta(days=1)
+        assert calculate_age(dob) == 18
+
+    def test_one_day_short_of_18(self):
+        """A person born one day after their 18th birthday is 17."""
+        today = datetime.now().date()
+        dob = today.replace(year=today.year - 18) + timedelta(days=1)
+        assert calculate_age(dob) == 17
+
+    def test_17_years_old(self):
+        """A person who is 17 years and 364 days old is 17."""
+        today = datetime.now().date()
+        dob = today.replace(year=today.year - 17) + timedelta(days=1)
+        assert calculate_age(dob) == 17
+
+    def test_65_years_old(self):
+        """A person who is 65 is correctly calculated."""
+        today = datetime.now().date()
+        dob = today.replace(year=today.year - 65)
+        assert calculate_age(dob) == 65
+
+    def test_newborn(self):
+        """A newborn is 0."""
+        today = datetime.now().date()
+        dob = today
+        assert calculate_age(dob) == 0
+
+    def test_birthday_not_yet_this_year(self):
+        """Person born Dec 31, 2000 — on Jan 1, 2025 they are still 24."""
+        dob = date(2000, 12, 31)
+        with patch("app.routers.auth.datetime") as mock_dt:
+            mock_dt.now.return_value = datetime(2025, 1, 1)
+            mock_dt.now.return_value.date.return_value = date(2025, 1, 1)
+            assert calculate_age(dob) == 24
+
+    def test_birthday_today(self):
+        """Person born Dec 31, 2000 — on Dec 31, 2024 they turn 24."""
+        dob = date(2000, 12, 31)
+        with patch("app.routers.auth.datetime") as mock_dt:
+            mock_dt.now.return_value = datetime(2024, 12, 31)
+            mock_dt.now.return_value.date.return_value = date(2024, 12, 31)
+            assert calculate_age(dob) == 24