migration: Remove CF account foreign key constraint

- CF accounts are managed in Admin Panel (separate database)
- cf_account_id is now just a reference ID, not a foreign key

backend/migrations/remove_cf_account_fk.sql

@@ -0,0 +1,11 @@
+-- Migration: Remove foreign key constraint on cf_account_id
+-- Reason: CF accounts are managed in Admin Panel (separate database)
+-- Date: 2026-01-12
+
+-- Drop the foreign key constraint
+ALTER TABLE domains 
+DROP CONSTRAINT IF EXISTS domains_cf_account_id_fkey;
+
+-- Add comment to explain why cf_account_id is not a foreign key
+COMMENT ON COLUMN domains.cf_account_id IS 'CF account ID from Admin Panel (not a foreign key - managed in separate database)';
+