fix(coderd/database): clean up org memberships when user is soft-deleted (#25149)

The soft-delete cleanup trigger (`delete_deleted_user_resources`)
removed `api_keys`, `user_links`, and `user_secrets` but left
`organization_members` rows intact. When a new user was created with a
previously-deleted user's email, both user IDs had org membership rows
in the same organization, producing duplicate-email members.

Extend the trigger to also delete `organization_members` for the
soft-deleted user. This cascades through the existing
`trigger_delete_group_members_on_org_member_delete`, which cleans up
group memberships automatically. The migration backfills by removing
zombie rows for already-deleted users.

Fixes ENG-831

> [!NOTE]
> 🤖 Generated by Coder Agents

<details>
<summary>Implementation notes</summary>

**Root cause**: `GetOrganizationIDsByMemberIDs` does not join on
`users.deleted = false`, so stale org membership rows for soft-deleted
users were visible to internal queries. Even the filtered queries
(`OrganizationMembers`, `PaginatedOrganizationMembers`) could surface
duplicate emails when a new active user reused a deleted user's email.

**What changed**:
- Migration 000491 extends `delete_deleted_user_resources()` to `DELETE
FROM organization_members WHERE user_id = OLD.id`
- Backfill removes existing zombie org memberships for soft-deleted
users
- `TestOrgMembersSoftDeleteTrigger` covers org membership removal, raw
row cleanup, and cascading group membership cleanup
</details>

coderd/database/dump.sql

@@ -768,6 +768,12 @@ BEGIN
 		-- does not remove the users row so the FK cascade never fires.
 		DELETE FROM user_secrets
 		WHERE user_id = OLD.id;
+
+		-- Remove their organization memberships.
+		-- This also triggers group membership cleanup via
+		-- trigger_delete_group_members_on_org_member_delete.
+		DELETE FROM organization_members
+		WHERE user_id = OLD.id;
 	END IF;
 	RETURN NEW;
 END;