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/urls.py

@@ -1,10 +1,19 @@
 from django.urls import path
 
-from .views import LeaderboardView, StreakCalendarView, UserHeartsView, UserSummaryView
+from .views import (
+    CharacterListView,
+    LeaderboardView,
+    SelectCharacterView,
+    StreakCalendarView,
+    UserHeartsView,
+    UserSummaryView,
+)
 
 urlpatterns = [
     path('user/summary', UserSummaryView.as_view(), name='user-summary'),
     path('user/streak/calendar', StreakCalendarView.as_view(), name='streak-calendar'),
     path('leaderboard', LeaderboardView.as_view(), name='leaderboard'),
     path('user/hearts', UserHeartsView.as_view(), name='user-hearts'),
+    path('characters', CharacterListView.as_view(), name='character-list'),
+    path('characters/select', SelectCharacterView.as_view(), name='character-select'),
 ]