feat: implement per-member reading progress tracking for shared books (#30)

Backend (apps/groups):
- ReadingGroup model: named groups linked to EBooks with creator tracking
- GroupMembership model: user-group association with member/admin roles
- MemberProgress model: per-member progress with section tracking,
  percentage, time_spent, privacy toggles, and device-position preservation
- ReadingGroupViewSet: CRUD, join/leave, members list, progress endpoints
- GET /api/groups/{id}/members/progress/ — all members' progress
  (public: section label only; own: full detail; private: stubbed)
- PATCH /api/groups/{id}/progress/ — update own progress with auto-
  percentage calculation from section boundaries
- GET /api/groups/{id}/progress/summary/ — admin dashboard with
  averages, started/finished counts, and per-member details
- Permissions: IsGroupMember, IsGroupAdmin

Frontend:
- Groups list page (/groups) with create modal and book selector
- Group detail page (/groups/:id) with progress and admin summary tabs
- Progress bar visualization per member with privacy-aware display
- Admin stat cards (total members, started, finished, avg progress, avg time)
- Navigation link from Library header

Shared:
- ReadingGroupSummary and MemberProgressPublic types
- API endpoint constants for groups routes

Closes #30

backend/apps/groups/migrations/0001_initial.py

@@ -0,0 +1,75 @@
+# Generated by Django 5.1.7 on 2026-06-20 19:21
+
+import django.db.models.deletion
+from django.conf import settings
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    initial = True
+
+    dependencies = [
+        ('books', '0003_readingprogress_epub_location'),
+        migrations.swappable_dependency(settings.AUTH_USER_MODEL),
+    ]
+
+    operations = [
+        migrations.CreateModel(
+            name='ReadingGroup',
+            fields=[
+                ('id', models.BigAutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID')),
+                ('name', models.CharField(db_index=True, max_length=256)),
+                ('description', models.TextField(blank=True, default='')),
+                ('created_at', models.DateTimeField(auto_now_add=True)),
+                ('updated_at', models.DateTimeField(auto_now=True)),
+                ('created_by', models.ForeignKey(on_delete=django.db.models.deletion.CASCADE, related_name='created_groups', to=settings.AUTH_USER_MODEL)),
+                ('ebook', models.ForeignKey(on_delete=django.db.models.deletion.CASCADE, related_name='reading_groups', to='books.ebook')),
+            ],
+            options={
+                'verbose_name': 'Reading Group',
+                'verbose_name_plural': 'Reading Groups',
+                'db_table': 'groups_reading_group',
+                'ordering': ['-created_at'],
+            },
+        ),
+        migrations.CreateModel(
+            name='GroupMembership',
+            fields=[
+                ('id', models.BigAutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID')),
+                ('role', models.CharField(choices=[('member', 'Member'), ('admin', 'Admin')], default='member', max_length=16)),
+                ('joined_at', models.DateTimeField(auto_now_add=True)),
+                ('user', models.ForeignKey(on_delete=django.db.models.deletion.CASCADE, related_name='group_memberships', to=settings.AUTH_USER_MODEL)),
+                ('group', models.ForeignKey(on_delete=django.db.models.deletion.CASCADE, related_name='memberships', to='groups.readinggroup')),
+            ],
+            options={
+                'verbose_name': 'Group Membership',
+                'verbose_name_plural': 'Group Memberships',
+                'db_table': 'groups_membership',
+                'ordering': ['joined_at'],
+                'unique_together': {('group', 'user')},
+            },
+        ),
+        migrations.CreateModel(
+            name='MemberProgress',
+            fields=[
+                ('id', models.BigAutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID')),
+                ('current_section', models.PositiveIntegerField(default=0)),
+                ('percentage', models.FloatField(default=0.0)),
+                ('time_spent_seconds', models.PositiveIntegerField(default=0)),
+                ('last_position', models.JSONField(blank=True, default=dict)),
+                ('is_public', models.BooleanField(default=True)),
+                ('updated_at', models.DateTimeField(auto_now=True)),
+                ('membership', models.OneToOneField(on_delete=django.db.models.deletion.CASCADE, related_name='progress', to='groups.groupmembership')),
+                ('user', models.ForeignKey(on_delete=django.db.models.deletion.CASCADE, related_name='member_progress', to=settings.AUTH_USER_MODEL)),
+                ('group', models.ForeignKey(on_delete=django.db.models.deletion.CASCADE, related_name='member_progress', to='groups.readinggroup')),
+            ],
+            options={
+                'verbose_name': 'Member Progress',
+                'verbose_name_plural': 'Member Progress',
+                'db_table': 'groups_member_progress',
+                'indexes': [models.Index(fields=['group', '-percentage'], name='groups_memb_group_i_42e333_idx'), models.Index(fields=['group', 'user'], name='groups_memb_group_i_f50793_idx')],
+                'unique_together': {('group', 'user')},
+            },
+        ),
+    ]