Registration: replace date-of-birth field with a simple 18+ confirmation

The register form collected and validated an actual date of birth just to
throw it away (backend never stored it beyond a nullable column). Per the
roadmap note: don't collect/store DOB right now, just get a client-side
"I am 18 or older" confirmation. RegisterRequest.date_of_birth (str) is now
over_18 (bool); the endpoint rejects over_18=false with 400 same as before.
calculate_age() is kept (unused by /register now) since real DOB collection
is coming back in a future pass and deleting it broke test_auth_age_gate.py's
import for no benefit.

Frontend: DOBEdit LineEdit replaced with an Over18Check CheckBox; api_client.gd's
register() now takes a bool instead of a date string. All Godot callers
(smoke_test_auth/e2e/world_list/main_menu_ui/main_menu_full_flow) and backend
test files updated to match. Full suite verified against a live backend:
61/61 pytest, 6/6 Godot smoke tests (33 individual assertions), all passing.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01M6rUEynmv2QmweocAQYARN

backend/tests/test_auth_age_gate.py

@@ -57,38 +57,38 @@ class TestCalculateAge:
         assert calculate_age(dob, today=date(2024, 12, 31)) == 24
 
 
-class TestRegisterRequestDateValidation:
-    """Tests for the RegisterRequest date_of_birth field validator."""
+class TestRegisterRequestOver18Field:
+    """Tests for RegisterRequest's over_18 field.
 
-    @pytest.mark.parametrize(
-        "dob",
-        [
-            "2000-01-01",
-            "1990-12-31",
-            "2004-02-29",  # leap day
-        ],
-    )
-    def test_valid_dates(self, dob):
-        """Valid YYYY-MM-DD dates pass validation."""
+    Replaces TestRegisterRequestDateValidation -- date_of_birth and its
+    field_validator were removed from RegisterRequest 2026-08-20 (interim
+    simplification: a plain client-side over_18 confirmation instead of
+    collecting/validating an actual date of birth; see calculate_age's
+    own docstring in app/routers/auth.py for why that function is still
+    here even though this endpoint no longer calls it). There's no date
+    format left to validate, so the equivalent coverage is just: the
+    field exists, is a real bool, and is required."""
+
+    def test_over_18_true_accepted(self):
         from app.routers.auth import RegisterRequest
 
-        req = RegisterRequest(username="u", email="u@example.com", password="p", date_of_birth=dob)
-        assert req.date_of_birth == dob
+        req = RegisterRequest(username="u", email="u@example.com", password="p", over_18=True)
+        assert req.over_18 is True
 
-    @pytest.mark.parametrize(
-        "dob",
-        [
-            "01-01-2000",
-            "2000/01/01",
-            "2000-13-01",
-            "2000-00-10",
-            "not-a-date",
-            "",
-        ],
-    )
-    def test_invalid_dates_rejected(self, dob):
-        """Invalid dates are rejected by the validator."""
+    def test_over_18_false_accepted(self):
+        """False is a valid value at the schema level -- the endpoint,
+        not the schema, is what rejects it (see
+        TestRegister::test_register_rejects_not_over_18 in
+        test_auth_endpoints.py)."""
         from app.routers.auth import RegisterRequest
 
-        with pytest.raises(ValueError):
-            RegisterRequest(username="u", email="u@example.com", password="p", date_of_birth=dob)
+        req = RegisterRequest(username="u", email="u@example.com", password="p", over_18=False)
+        assert req.over_18 is False
+
+    def test_over_18_missing_rejected(self):
+        """over_18 has no default -- omitting it entirely is a validation error."""
+        from app.routers.auth import RegisterRequest
+        from pydantic import ValidationError
+
+        with pytest.raises(ValidationError):
+            RegisterRequest(username="u", email="u@example.com", password="p")