Add variable amount bills

- Migration 002: variable_amount boolean column on bills (default false)
- Bills form: 'Variable amount' checkbox; amount field becomes optional
  'Typical amount' when checked; table shows 'varies (~$X)' and a 〜 badge
- Paycheck view: variable bills show a pencil edit button to enter the
  month's actual amount, stored as amount_override on paycheck_bills
- New PATCH /api/paycheck-bills/:id/amount endpoint
- Lazy generation still works: setting an amount on a virtual paycheck
  generates it first then saves the override

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

server/src/routes/paychecks.js

@@ -56,7 +56,7 @@ async function buildVirtualPaychecks(year, month) {
     const payDate = `${year}-${pad2(month)}-${pad2(day ?? 1)}`;
 
     const billsResult = await pool.query(
-      `SELECT id, name, amount, due_day, category
+      `SELECT id, name, amount, due_day, category, variable_amount
        FROM bills WHERE active = TRUE AND assigned_paycheck = $1
        ORDER BY due_day, name`,
       [num]
@@ -79,6 +79,7 @@ async function buildVirtualPaychecks(year, month) {
         effective_amount: b.amount,
         due_day: b.due_day,
         category: b.category,
+        variable_amount: b.variable_amount,
         paid: false,
         paid_at: null,
       })),
@@ -162,6 +163,7 @@ async function fetchPaychecksForMonth(year, month) {
               pb.bill_id,
               b.name,
               b.amount,
+              b.variable_amount,
               pb.amount_override,
               CASE WHEN pb.amount_override IS NOT NULL THEN pb.amount_override ELSE b.amount END AS effective_amount,
               b.due_day,
@@ -196,6 +198,7 @@ async function fetchPaychecksForMonth(year, month) {
         bill_id: b.bill_id,
         name: b.name,
         amount: b.amount,
+        variable_amount: b.variable_amount,
         amount_override: b.amount_override,
         effective_amount: b.effective_amount,
         due_day: b.due_day,
@@ -303,6 +306,36 @@ router.patch('/paychecks/:id', async (req, res) => {
   }
 });
 
+// PATCH /api/paycheck-bills/:id/amount — set amount_override for a variable bill
+router.patch('/paycheck-bills/:id/amount', async (req, res) => {
+  const id = parseInt(req.params.id, 10);
+  if (isNaN(id)) {
+    return res.status(400).json({ error: 'Invalid id' });
+  }
+
+  const { amount } = req.body;
+  if (amount == null || isNaN(parseFloat(amount))) {
+    return res.status(400).json({ error: 'amount is required' });
+  }
+
+  try {
+    const result = await pool.query(
+      `UPDATE paycheck_bills SET amount_override = $1 WHERE id = $2
+       RETURNING id, amount_override`,
+      [parseFloat(amount), id]
+    );
+
+    if (result.rows.length === 0) {
+      return res.status(404).json({ error: 'paycheck_bill not found' });
+    }
+
+    res.json(result.rows[0]);
+  } catch (err) {
+    console.error('PATCH /api/paycheck-bills/:id/amount error:', err);
+    res.status(500).json({ error: 'Failed to update amount' });
+  }
+});
+
 // PATCH /api/paycheck-bills/:id/paid
 router.patch('/paycheck-bills/:id/paid', async (req, res) => {
   const id = parseInt(req.params.id, 10);