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/schemas/group.py

@@ -0,0 +1,37 @@
+from datetime import datetime
+
+from pydantic import BaseModel, Field
+
+
+class GroupCreate(BaseModel):
+    name: str = Field(..., min_length=1, max_length=128)
+    description: str | None = Field(None, max_length=512)
+
+
+class GroupUpdate(BaseModel):
+    name: str | None = Field(None, min_length=1, max_length=128)
+    description: str | None = Field(None, max_length=512)
+
+
+class GroupMemberOut(BaseModel):
+    id: str
+    email: str
+    full_name: str | None
+    is_active: bool
+    joined_at: datetime
+
+    model_config = {"from_attributes": True}
+
+
+class GroupOut(BaseModel):
+    id: str
+    name: str
+    description: str | None
+    created_at: datetime
+    member_count: int = 0
+
+    model_config = {"from_attributes": True}
+
+
+class GroupDetailOut(GroupOut):
+    members: list[GroupMemberOut] = []