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/models.py

@@ -0,0 +1,138 @@
+from django.conf import settings
+from django.db import models
+
+
+class ReadingGroup(models.Model):
+    """A group of users reading the same book together."""
+
+    name = models.CharField(max_length=256, db_index=True)
+    ebook = models.ForeignKey(
+        "books.EBook",
+        on_delete=models.CASCADE,
+        related_name="reading_groups",
+    )
+    created_by = models.ForeignKey(
+        settings.AUTH_USER_MODEL,
+        on_delete=models.CASCADE,
+        related_name="created_groups",
+    )
+    description = models.TextField(blank=True, default="")
+    created_at = models.DateTimeField(auto_now_add=True)
+    updated_at = models.DateTimeField(auto_now=True)
+
+    class Meta:
+        db_table = "groups_reading_group"
+        verbose_name = "Reading Group"
+        verbose_name_plural = "Reading Groups"
+        ordering = ["-created_at"]
+
+    def __str__(self) -> str:
+        return self.name
+
+    @property
+    def total_sections(self) -> int:
+        return self.ebook.chapters.count()
+
+
+class GroupMembership(models.Model):
+    class Role(models.TextChoices):
+        MEMBER = "member", "Member"
+        ADMIN = "admin", "Admin"
+
+    group = models.ForeignKey(
+        ReadingGroup,
+        on_delete=models.CASCADE,
+        related_name="memberships",
+    )
+    user = models.ForeignKey(
+        settings.AUTH_USER_MODEL,
+        on_delete=models.CASCADE,
+        related_name="group_memberships",
+    )
+    role = models.CharField(
+        max_length=16,
+        choices=Role.choices,
+        default=Role.MEMBER,
+    )
+    joined_at = models.DateTimeField(auto_now_add=True)
+
+    class Meta:
+        db_table = "groups_membership"
+        verbose_name = "Group Membership"
+        verbose_name_plural = "Group Memberships"
+        unique_together = [("group", "user")]
+        ordering = ["joined_at"]
+
+    def __str__(self) -> str:
+        return f"{self.user} in {self.group.name} ({self.role})"
+
+
+class MemberProgress(models.Model):
+    """Per-member reading progress within a reading group."""
+
+    membership = models.OneToOneField(
+        GroupMembership,
+        on_delete=models.CASCADE,
+        related_name="progress",
+    )
+    group = models.ForeignKey(
+        ReadingGroup,
+        on_delete=models.CASCADE,
+        related_name="member_progress",
+    )
+    user = models.ForeignKey(
+        settings.AUTH_USER_MODEL,
+        on_delete=models.CASCADE,
+        related_name="member_progress",
+    )
+    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)
+
+    class Meta:
+        db_table = "groups_member_progress"
+        verbose_name = "Member Progress"
+        verbose_name_plural = "Member Progress"
+        unique_together = [("group", "user")]
+        indexes = [
+            models.Index(fields=["group", "-percentage"]),
+            models.Index(fields=["group", "user"]),
+        ]
+
+    def __str__(self) -> str:
+        return f"{self.user} — {self.group.name} ({self.percentage:.0f}%)"
+
+    def total_sections(self) -> int:
+        return self.group.total_sections
+
+    def update_progress(
+        self,
+        current_section: int,
+        percentage: float | None = None,
+        time_spent_delta: int = 0,
+        last_position: dict | None = None,
+    ) -> None:
+        """Update progress with computed percentage if not provided."""
+        self.current_section = current_section
+        if percentage is not None:
+            self.percentage = min(100.0, max(0.0, percentage))
+        elif (ts := self.total_sections()) > 0:
+            self.percentage = min(100.0, (current_section / ts) * 100.0)
+        else:
+            self.percentage = 0.0
+        if time_spent_delta > 0:
+            self.time_spent_seconds += time_spent_delta
+        if last_position is not None:
+            self.last_position = last_position
+        self.save(
+            update_fields=[
+                "current_section",
+                "percentage",
+                "time_spent_seconds",
+                "last_position",
+                "updated_at",
+            ]
+        )