Correct the policy count in the places that quote it

Six comments and doc lines put the number of RLS policies at 58. It is
21 — counted from pg_policy while building the data catalogue. The
figure appears in load-bearing prose ("all 58 policies call
is_hr_user()", "all 58 policies stay unchanged"), where being wrong by a
factor of three invites someone to go looking for the missing thirty-
seven.

The two occurrences inside supabase/migrations/ stay as they are. That
file already ran against the database; its comments record what was
believed at the time, and editing them would make the file differ from
what was applied for no gain.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

lib/auth/session.ts

@@ -11,7 +11,7 @@ import { auth } from "@/auth";
 // zwischen beiden macht app_upsert_user() bei der Anmeldung, und sie
 // übernimmt für eine bereits bekannte Adresse die vorhandene profiles.id.
 // Deshalb passt die Kennung weiterhin auf das, was app_current_user_id() in
-// der Datenbank erwartet, und die 58 RLS-Policies merken vom Wechsel nichts.
+// der Datenbank erwartet, und die 21 RLS-Policies merken vom Wechsel nichts.
 
 export async function currentUserId(): Promise<string | null> {
   const session = await auth();