Fix variable bill amount lost on paid toggle; revert gross/net protection

- PATCH paycheck-bills/:id/paid: variable bills now preserve amount_override
  rather than overwriting it with b.amount (which may be null/0). Fixed bills
  continue to lock in b.amount on paid and clear on unpaid.
- generatePaychecks: revert gross/net protection — refresh always updates
  gross/net from current settings as originally intended.
- CLAUDE.md: remove gross/net protection note; add td approve sub-agent rule.

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

server/src/routes/paychecks.js

@@ -159,20 +159,8 @@ async function generatePaychecks(year, month) {
          VALUES ($1, $2, $3, $4, $5, $6)
          ON CONFLICT (period_year, period_month, paycheck_number)
          DO UPDATE SET pay_date = EXCLUDED.pay_date,
-                       gross = CASE
-                         WHEN EXISTS (
-                           SELECT 1 FROM paycheck_bills pb
-                           WHERE pb.paycheck_id = paychecks.id AND pb.paid = TRUE
-                         ) THEN paychecks.gross
-                         ELSE EXCLUDED.gross
-                       END,
-                       net = CASE
-                         WHEN EXISTS (
-                           SELECT 1 FROM paycheck_bills pb
-                           WHERE pb.paycheck_id = paychecks.id AND pb.paid = TRUE
-                         ) THEN paychecks.net
-                         ELSE EXCLUDED.net
-                       END
+                       gross = EXCLUDED.gross,
+                       net = EXCLUDED.net
          RETURNING id`,
         [year, month, num, payDate, gross || 0, net || 0]
       );
@@ -474,7 +462,11 @@ router.patch('/paycheck-bills/:id/paid', async (req, res) => {
       `UPDATE paycheck_bills pb
        SET paid = $1,
            paid_at = CASE WHEN $1 THEN NOW() ELSE NULL END,
-           amount_override = CASE WHEN $1 THEN b.amount ELSE NULL END
+           amount_override = CASE
+             WHEN b.variable_amount THEN pb.amount_override
+             WHEN $1 THEN b.amount
+             ELSE NULL
+           END
        FROM bills b
        WHERE pb.bill_id = b.id AND pb.id = $2
        RETURNING pb.id, pb.paid, pb.paid_at, pb.amount_override`,