Lock bill amount_override when marked paid, clear on unpaid

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

server/src/routes/paychecks.js

@@ -246,11 +246,13 @@ router.patch('/paycheck-bills/:id/paid', async (req, res) => {
 
   try {
     const result = await pool.query(
-      `UPDATE paycheck_bills
+      `UPDATE paycheck_bills pb
        SET paid = $1,
-           paid_at = CASE WHEN $1 THEN NOW() ELSE NULL END
+           paid_at = CASE WHEN $1 THEN NOW() ELSE NULL END,
-       WHERE id = $2
+           amount_override = CASE WHEN $1 THEN b.amount ELSE NULL END
-       RETURNING id, paid, paid_at`,
+       FROM bills b
+       WHERE pb.bill_id = b.id AND pb.id = $2
+       RETURNING pb.id, pb.paid, pb.paid_at, pb.amount_override`,
       [paid, id]
     );