feat: implement group creation and management (US #28)

Backend:
- Add groups Django app with models: Group, GroupMember, GroupInvite, JoinRequest
- Create serializers with business rule validation
- Implement GroupViewSet with full CRUD + custom actions (members, invites, roles, leave, join requests)
- Add JoinGroupViewSet for invite-based joining flow
- Register app in Django config and URL routing

Frontend:
- Add shared types for groups to @cloud-reader/shared
- Create groups API client (groupsApi)
- Build GroupsListPage, GroupDetailPage (member mgmt, invites, role transfer)
- Build CreateGroupPage and JoinGroupPage
- Add lazy-loaded routes to App.tsx with ProtectedRoute
- Add navigation links to Library header

Ref: #28

backend/apps/groups/models.py

@@ -0,0 +1,129 @@
+import uuid
+
+from django.conf import settings
+from django.db import models
+
+
+class GroupRole(models.TextChoices):
+    ADMIN = "admin", "Admin"
+    MEMBER = "member", "Member"
+
+
+class JoinRequestStatus(models.TextChoices):
+    PENDING = "pending", "Pending"
+    APPROVED = "approved", "Approved"
+    REJECTED = "rejected", "Rejected"
+
+
+class Group(models.Model):
+    name = models.CharField(max_length=256, db_index=True)
+    description = models.TextField(blank=True, default="")
+    created_by = models.ForeignKey(
+        settings.AUTH_USER_MODEL,
+        on_delete=models.CASCADE,
+        related_name="created_groups",
+    )
+    created_at = models.DateTimeField(auto_now_add=True)
+    updated_at = models.DateTimeField(auto_now=True)
+
+    class Meta:
+        db_table = "groups_group"
+        verbose_name = "Group"
+        verbose_name_plural = "Groups"
+        ordering = ["-created_at"]
+
+    def __str__(self) -> str:
+        return self.name
+
+
+class GroupMember(models.Model):
+    group = models.ForeignKey(
+        Group,
+        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=GroupRole.choices,
+        default=GroupRole.MEMBER,
+    )
+    joined_at = models.DateTimeField(auto_now_add=True)
+
+    class Meta:
+        db_table = "groups_member"
+        verbose_name = "Group Member"
+        verbose_name_plural = "Group Members"
+        ordering = ["joined_at"]
+        unique_together = [("group", "user")]
+
+    def __str__(self) -> str:
+        return f"{self.user} in {self.group} ({self.role})"
+
+
+class GroupInvite(models.Model):
+    group = models.ForeignKey(
+        Group,
+        on_delete=models.CASCADE,
+        related_name="invites",
+    )
+    created_by = models.ForeignKey(
+        settings.AUTH_USER_MODEL,
+        on_delete=models.CASCADE,
+        related_name="created_invites",
+    )
+    code = models.UUIDField(default=uuid.uuid4, unique=True, editable=False, db_index=True)
+    max_uses = models.PositiveIntegerField(default=0, help_text="0 = unlimited")
+    use_count = models.PositiveIntegerField(default=0)
+    is_active = models.BooleanField(default=True, db_index=True)
+    created_at = models.DateTimeField(auto_now_add=True)
+
+    class Meta:
+        db_table = "groups_invite"
+        verbose_name = "Group Invite"
+        verbose_name_plural = "Group Invites"
+        ordering = ["-created_at"]
+
+    def __str__(self) -> str:
+        return f"Invite for {self.group.name} ({self.code})"
+
+
+class JoinRequest(models.Model):
+    group = models.ForeignKey(
+        Group,
+        on_delete=models.CASCADE,
+        related_name="join_requests",
+    )
+    user = models.ForeignKey(
+        settings.AUTH_USER_MODEL,
+        on_delete=models.CASCADE,
+        related_name="join_requests",
+    )
+    invite = models.ForeignKey(
+        GroupInvite,
+        on_delete=models.SET_NULL,
+        null=True,
+        blank=True,
+        related_name="join_requests",
+    )
+    status = models.CharField(
+        max_length=16,
+        choices=JoinRequestStatus.choices,
+        default=JoinRequestStatus.PENDING,
+        db_index=True,
+    )
+    created_at = models.DateTimeField(auto_now_add=True)
+
+    class Meta:
+        db_table = "groups_join_request"
+        verbose_name = "Join Request"
+        verbose_name_plural = "Join Requests"
+        ordering = ["-created_at"]
+        unique_together = [("group", "user")]
+
+    def __str__(self) -> str:
+        return f"{self.user} → {self.group.name} ({self.status})"