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/migrations/0002_character_userstats_selected_character_id.py

@@ -0,0 +1,32 @@
+# Generated by Django 5.2.17 on 2026-09-06 10:17
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('progress', '0001_initial'),
+    ]
+
+    operations = [
+        migrations.CreateModel(
+            name='Character',
+            fields=[
+                ('id', models.BigAutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID')),
+                ('slug', models.SlugField(unique=True)),
+                ('name', models.CharField(max_length=50)),
+                ('icon', models.CharField(max_length=100)),
+                ('unlock_xp', models.PositiveIntegerField(default=0)),
+                ('order', models.PositiveIntegerField(default=0)),
+            ],
+            options={
+                'ordering': ['order', 'unlock_xp'],
+            },
+        ),
+        migrations.AddField(
+            model_name='userstats',
+            name='selected_character_id',
+            field=models.IntegerField(blank=True, null=True),
+        ),
+    ]