add gamification characters (Duolingo-style mascot catalog)

New progress.Character model: an admin-managed catalog (same pattern as
subscription.Plan) of selectable mascots, each unlocked once the user's
XP crosses unlock_xp. No separate ownership table yet — unlocking is a
pure function of UserStats.xp, kept intentionally minimal since this is
the first of an incremental rollout the senior asked for.

- GET /api/characters — catalog with per-user unlocked/selected flags
- POST /api/characters/select — picks an active character, rejecting
  ones the user hasn't unlocked yet
- UserStats.selected_character_id tracks the pick
- seed_test_data now seeds 4 sample characters (owl/fox/turtle/lion)
  for local/staging testing

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

progress/tests.py

@@ -7,7 +7,7 @@ from rest_framework.test import APIClient
 from accounts.authentication import issue_access_token
 from accounts.models import User
 
-from .models import HEART_REFILL_INTERVAL, UserHearts
+from .models import HEART_REFILL_INTERVAL, Character, UserHearts, UserStats
 
 
 class UserHeartsRegenTests(TestCase):
@@ -60,3 +60,72 @@ class UserHeartsRegenTests(TestCase):
         response = self._get_hearts()
         self.assertEqual(response.data['currentHearts'], 5)
         self.assertIsNone(response.data['nextRefillAt'])
+
+
+class CharacterGamificationTests(TestCase):
+    """کاتالوگِ کاراکترها (مثلِ ماسکات‌های Duolingo): باز/بسته‌بودن تابعی از
+    XP کاربره، و فقط کاراکترِ باز‌شده قابلِ انتخاب‌شدنه."""
+
+    def setUp(self):
+        self.client = APIClient()
+        self.user = User.objects.create(device_id='device-characters-1', is_guest=True, name='')
+        self.token = issue_access_token(self.user.id)
+
+        self.owl = Character.objects.create(slug='owl', name='جغد', icon='owl', unlock_xp=0, order=1)
+        self.fox = Character.objects.create(slug='fox', name='روباه', icon='fox', unlock_xp=100, order=2)
+
+    def _list(self):
+        return self.client.get('/api/characters', HTTP_AUTHORIZATION=f'Bearer {self.token}')
+
+    def _select(self, character_id):
+        return self.client.post(
+            '/api/characters/select', {'characterId': str(character_id)},
+            format='json', HTTP_AUTHORIZATION=f'Bearer {self.token}',
+        )
+
+    def test_list_reports_locked_and_unlocked_by_xp(self):
+        UserStats.objects.create(user_id=self.user.id, xp=50)
+
+        response = self._list()
+
+        self.assertEqual(response.status_code, 200)
+        by_slug = {c['slug']: c for c in response.data['characters']}
+        self.assertTrue(by_slug['owl']['unlocked'])
+        self.assertFalse(by_slug['fox']['unlocked'])
+
+    def test_list_with_no_stats_yet_only_zero_xp_characters_unlocked(self):
+        response = self._list()
+
+        self.assertEqual(response.status_code, 200)
+        by_slug = {c['slug']: c for c in response.data['characters']}
+        self.assertTrue(by_slug['owl']['unlocked'])
+        self.assertFalse(by_slug['fox']['unlocked'])
+
+    def test_can_select_unlocked_character(self):
+        UserStats.objects.create(user_id=self.user.id, xp=10)
+
+        response = self._select(self.owl.id)
+
+        self.assertEqual(response.status_code, 200)
+        self.assertTrue(response.data['selected'])
+        stats = UserStats.objects.get(user_id=self.user.id)
+        self.assertEqual(stats.selected_character_id, self.owl.id)
+
+    def test_cannot_select_locked_character(self):
+        UserStats.objects.create(user_id=self.user.id, xp=10)
+
+        response = self._select(self.fox.id)
+
+        self.assertEqual(response.status_code, 400)
+        stats = UserStats.objects.get(user_id=self.user.id)
+        self.assertIsNone(stats.selected_character_id)
+
+    def test_selection_reflected_in_list(self):
+        UserStats.objects.create(user_id=self.user.id, xp=10)
+        self._select(self.owl.id)
+
+        response = self._list()
+
+        by_slug = {c['slug']: c for c in response.data['characters']}
+        self.assertTrue(by_slug['owl']['selected'])
+        self.assertFalse(by_slug['fox']['selected'])