Add Groups management and split Admin navigation

- New backend: Group + GroupMembership models, schemas, CRUD router at
  /api/admin/groups (list, create, get detail, update, delete, add/remove members)
- New Alembic migration: groups and group_memberships tables
- Frontend: Admin sidebar item is now an expandable accordion with
  Users and Groups sub-items; AdminPage redirects to /admin/users;
  new AdminUsersPage and AdminGroupsPage with inline member management panel
- API client: 7 new group functions + TypeScript types

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/models/__init__.py

@@ -1,4 +1,5 @@
+from app.models.group import Group, GroupMembership
 from app.models.profile import Profile
 from app.models.user import User
 
-__all__ = ["User", "Profile"]
+__all__ = ["User", "Profile", "Group", "GroupMembership"]

backend/app/models/group.py

@@ -0,0 +1,44 @@
+import uuid
+from datetime import datetime, timezone
+
+from sqlalchemy import DateTime, ForeignKey, String, UniqueConstraint
+from sqlalchemy.orm import Mapped, mapped_column, relationship
+
+from app.database import Base
+
+
+class Group(Base):
+    __tablename__ = "groups"
+
+    id: Mapped[str] = mapped_column(String, primary_key=True, default=lambda: str(uuid.uuid4()))
+    name: Mapped[str] = mapped_column(String(128), unique=True, nullable=False, index=True)
+    description: Mapped[str | None] = mapped_column(String(512), nullable=True)
+    created_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True),
+        default=lambda: datetime.now(timezone.utc),
+        nullable=False,
+    )
+
+    memberships: Mapped[list["GroupMembership"]] = relationship(
+        "GroupMembership", back_populates="group", cascade="all, delete-orphan"
+    )
+
+
+class GroupMembership(Base):
+    __tablename__ = "group_memberships"
+    __table_args__ = (UniqueConstraint("group_id", "user_id", name="uq_group_user"),)
+
+    id: Mapped[str] = mapped_column(String, primary_key=True, default=lambda: str(uuid.uuid4()))
+    group_id: Mapped[str] = mapped_column(
+        String, ForeignKey("groups.id", ondelete="CASCADE"), nullable=False, index=True
+    )
+    user_id: Mapped[str] = mapped_column(
+        String, ForeignKey("users.id", ondelete="CASCADE"), nullable=False, index=True
+    )
+    joined_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True),
+        default=lambda: datetime.now(timezone.utc),
+        nullable=False,
+    )
+
+    group: Mapped["Group"] = relationship("Group", back_populates="memberships")