[PR 1] Add experimental Custom Roles foundation (#7847)

migrations/mysql/2026-10-10-120000_add_custom_role_permissions/down.sql

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+-- Dropping the permissions would silently take them away from the Custom members, who would also keep a role (4) the
+-- versions before this one don't know. So while there are any, this fails on the duplicate key below, before anything
+-- is dropped: change their role first, or restore the backup taken before the upgrade.
+-- A failed attempt leaves the temporary table on this connection. Clear it before retrying.
+DROP TEMPORARY TABLE IF EXISTS custom_role_downgrade;
+CREATE TEMPORARY TABLE custom_role_downgrade (blocked_by VARCHAR(255) NOT NULL PRIMARY KEY);
+INSERT INTO custom_role_downgrade VALUES ('Custom members: change their role first, or restore the backup taken before the upgrade');
+INSERT INTO custom_role_downgrade
+    SELECT 'Custom members: change their role first, or restore the backup taken before the upgrade'
+    FROM users_organizations WHERE atype = 4 LIMIT 1;
+DROP TEMPORARY TABLE custom_role_downgrade;
+
+ALTER TABLE users_organizations
+    DROP COLUMN manage_users,
+    DROP COLUMN manage_groups,
+    DROP COLUMN manage_policies,
+    DROP COLUMN create_new_collections,
+    DROP COLUMN edit_any_collection,
+    DROP COLUMN delete_any_collection,
+    DROP COLUMN access_event_logs,
+    DROP COLUMN access_import_export,
+    DROP COLUMN access_reports;

migrations/mysql/2026-10-10-120000_add_custom_role_permissions/up.sql

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+-- The granular permissions of the Custom member role. They only take effect on a Custom membership
+-- (atype 4) while EXPERIMENTAL_CUSTOM_ROLES is enabled, and every existing membership starts without any.
+-- A single statement, so an interrupted upgrade cannot leave only some of the columns behind.
+ALTER TABLE users_organizations
+    ADD COLUMN manage_users BOOLEAN NOT NULL DEFAULT FALSE,
+    ADD COLUMN manage_groups BOOLEAN NOT NULL DEFAULT FALSE,
+    ADD COLUMN manage_policies BOOLEAN NOT NULL DEFAULT FALSE,
+    ADD COLUMN create_new_collections BOOLEAN NOT NULL DEFAULT FALSE,
+    ADD COLUMN edit_any_collection BOOLEAN NOT NULL DEFAULT FALSE,
+    ADD COLUMN delete_any_collection BOOLEAN NOT NULL DEFAULT FALSE,
+    ADD COLUMN access_event_logs BOOLEAN NOT NULL DEFAULT FALSE,
+    ADD COLUMN access_import_export BOOLEAN NOT NULL DEFAULT FALSE,
+    ADD COLUMN access_reports BOOLEAN NOT NULL DEFAULT FALSE;