feat: category scopes, group-admin role, and permission model

- Three category scopes: personal / group / system (watch)
- PascalCase-with-dashes naming convention enforced at backend + frontend
- is_group_admin flag on GroupMembership; PATCH endpoint for admins to toggle it
- Categories router: scope-based list/create/rename/delete with _check_can_manage_cat
- Documents router: delete uses is_admin + can_delete share flag + group-admin check; remove_category requires doc ownership; assign_category accepts group/system categories
- Proxy layers inject x-user-is-admin and x-user-admin-groups headers
- Frontend: ManageCategoriesDialog grouped by scope with lock icons; SourcePanel scope picker + client-side name validation; AdminGroupsPage group-admin checkbox

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

backend/alembic/versions/e1f2a3b4c5d6_add_group_member_is_admin.py

@@ -0,0 +1,32 @@
+"""add is_group_admin to group_memberships
+
+Revision ID: e1f2a3b4c5d6
+Revises: dd6ad2f2c211
+Create Date: 2026-04-18
+
+"""
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+from alembic import op
+
+revision: str = "e1f2a3b4c5d6"
+down_revision: Union[str, None] = "dd6ad2f2c211"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.add_column(
+        "group_memberships",
+        sa.Column(
+            "is_group_admin",
+            sa.Boolean(),
+            nullable=False,
+            server_default=sa.text("false"),
+        ),
+    )
+
+
+def downgrade() -> None:
+    op.drop_column("group_memberships", "is_group_admin")