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/serializers.py

@@ -40,3 +40,26 @@ class UserHeartsResponseSerializer(serializers.Serializer):
     maxHearts = serializers.IntegerField()
     nextRefillAt = serializers.DateTimeField(allow_null=True)
     isUnlimited = serializers.BooleanField()
+
+
+class CharacterSerializer(serializers.Serializer):
+    characterId = serializers.CharField()
+    slug = serializers.CharField()
+    name = serializers.CharField()
+    icon = serializers.CharField()
+    unlockXp = serializers.IntegerField()
+    unlocked = serializers.BooleanField()
+    selected = serializers.BooleanField()
+
+
+class CharacterListResponseSerializer(serializers.Serializer):
+    characters = CharacterSerializer(many=True)
+
+
+class SelectCharacterRequestSerializer(serializers.Serializer):
+    characterId = serializers.CharField()
+
+
+class SelectCharacterResponseSerializer(serializers.Serializer):
+    characterId = serializers.CharField()
+    selected = serializers.BooleanField()