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/admin.py

@@ -0,0 +1,23 @@
+from django.contrib import admin
+
+from apps.groups.models import GroupMembership, MemberProgress, ReadingGroup
+
+
+@admin.register(ReadingGroup)
+class ReadingGroupAdmin(admin.ModelAdmin):
+    list_display = ["name", "ebook", "created_by", "created_at"]
+    search_fields = ["name", "ebook__title", "created_by__email"]
+
+
+@admin.register(GroupMembership)
+class GroupMembershipAdmin(admin.ModelAdmin):
+    list_display = ["group", "user", "role", "joined_at"]
+    list_filter = ["role"]
+    search_fields = ["user__email", "group__name"]
+
+
+@admin.register(MemberProgress)
+class MemberProgressAdmin(admin.ModelAdmin):
+    list_display = ["user", "group", "current_section", "percentage", "time_spent_seconds", "is_public", "updated_at"]
+    list_filter = ["is_public"]
+    search_fields = ["user__email", "group__name"]