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/admin.py

@@ -1,11 +1,17 @@
 from django.contrib import admin
 
-from .models import ActiveDay, LeaderboardEntry, StreakRecord, UserHearts, UserStats
+from .models import ActiveDay, Character, LeaderboardEntry, StreakRecord, UserHearts, UserStats
 
 
 @admin.register(UserStats)
 class UserStatsAdmin(admin.ModelAdmin):
-    list_display = ('id', 'user_id', 'xp', 'coins', 'progress_percent')
+    list_display = ('id', 'user_id', 'xp', 'coins', 'progress_percent', 'selected_character_id')
+
+
+@admin.register(Character)
+class CharacterAdmin(admin.ModelAdmin):
+    list_display = ('id', 'name', 'slug', 'icon', 'unlock_xp', 'order')
+    ordering = ('order', 'unlock_xp')
 
 
 @admin.register(LeaderboardEntry)