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/config/urls.py

@@ -8,6 +8,7 @@ urlpatterns = [
     path("api/books/", include("apps.books.urls")),
     path("api/annotations/", include("apps.annotations.urls")),
     path("api/reader/", include("apps.reader.urls")),
+    path("api/groups/", include("apps.groups.urls")),
 ]
 
 if settings.DEBUG: