test(budget): settled_at through the HTTP pipe

The e2e settlement round trip now sends settled_at on create and update
and reads it back from the column, including a PUT that clears it to
NULL. The service test covers the clear branch for null and the empty
string the date picker sends.

server/tests/e2e/budget.e2e.test.ts

@@ -228,22 +228,41 @@ describe('Budget e2e (real auth guard + temp SQLite, real budget SQL)', () => {
     }
   });
 
-  it('200 on settlement update with permission, persisting the new amount', async () => {
+  it('200 on settlement update with permission, persisting the new amount and day', async () => {
     const created = await request(server)
       .post(`/api/trips/${tripId}/budget/settlements`)
       .set('Cookie', sessionCookie(1))
-      .send({ from_user_id: 2, to_user_id: 1, amount: 10 });
+      .send({ from_user_id: 2, to_user_id: 1, amount: 10, settled_at: '2026-01-05' });
+    expect(created.status).toBe(201);
+    expect(created.body.settlement.settled_at).toBe('2026-01-05');
+
+    const res = await request(server)
+      .put(`/api/trips/${tripId}/budget/settlements/${created.body.settlement.id}`)
+      .set('Cookie', sessionCookie(1))
+      .send({ from_user_id: 2, to_user_id: 1, amount: 15, settled_at: '2026-01-09' });
+    expect(res.status).toBe(200);
+    expect(res.body.settlement).toMatchObject({ id: created.body.settlement.id, from_user_id: 2, to_user_id: 1, amount: 15, settled_at: '2026-01-09' });
+
+    const row = db.prepare('SELECT amount, settled_at FROM budget_settlements WHERE id = ?').get(created.body.settlement.id);
+    expect(row).toEqual({ amount: 15, settled_at: '2026-01-09' });
+  });
+
+  it('200 on settlement update that clears the day, leaving NULL in the column', async () => {
+    const created = await request(server)
+      .post(`/api/trips/${tripId}/budget/settlements`)
+      .set('Cookie', sessionCookie(1))
+      .send({ from_user_id: 2, to_user_id: 1, amount: 10, settled_at: '2026-01-05' });
     expect(created.status).toBe(201);
 
     const res = await request(server)
       .put(`/api/trips/${tripId}/budget/settlements/${created.body.settlement.id}`)
       .set('Cookie', sessionCookie(1))
-      .send({ from_user_id: 2, to_user_id: 1, amount: 15 });
+      .send({ from_user_id: 2, to_user_id: 1, amount: 10, settled_at: null });
     expect(res.status).toBe(200);
-    expect(res.body.settlement).toMatchObject({ id: created.body.settlement.id, from_user_id: 2, to_user_id: 1, amount: 15 });
+    expect(res.body.settlement.settled_at).toBeNull();
 
-    const row = db.prepare('SELECT amount FROM budget_settlements WHERE id = ?').get(created.body.settlement.id);
-    expect(row).toEqual({ amount: 15 });
+    const row = db.prepare('SELECT settled_at FROM budget_settlements WHERE id = ?').get(created.body.settlement.id);
+    expect(row).toEqual({ settled_at: null });
   });
 
   it('404 on settlement update when it does not exist', async () => {

server/tests/unit/nest/budget.service.db.test.ts

@@ -790,6 +790,26 @@ describe('post-fold quirk fixes', () => {
     expect(untouched!.settled_at).toBe('2026-01-09');
   });
 
+  it('BUDGET-SVC-DB-043: clearing settled_at stores NULL, whether it arrives as null or an empty string', () => {
+    const { user: alice } = createUser(testDb, { username: 'alice' });
+    const { user: bob } = createUser(testDb, { username: 'bob' });
+    const trip = createTrip(testDb, alice.id);
+    const parties = { from_user_id: alice.id, to_user_id: bob.id, amount: 20 };
+
+    // The date picker's clear button sends '' and the contract allows null; both
+    // mean "no day of its own", never a stored empty string.
+    const blank = budget.insertSettlement(trip.id, { ...parties, settled_at: '' }, alice.id);
+    expect(blank!.settled_at).toBeNull();
+
+    const dated = budget.insertSettlement(trip.id, { ...parties, settled_at: '2026-01-05' }, alice.id);
+    expect(budget.applySettlementUpdate(dated!.id, trip.id, { ...parties, settled_at: null })!.settled_at).toBeNull();
+
+    budget.applySettlementUpdate(dated!.id, trip.id, { ...parties, settled_at: '2026-01-05' });
+    expect(budget.applySettlementUpdate(dated!.id, trip.id, { ...parties, settled_at: '' })!.settled_at).toBeNull();
+    const row = testDb.prepare('SELECT settled_at FROM budget_settlements WHERE id = ?').get(dated!.id) as { settled_at: string | null };
+    expect(row.settled_at).toBeNull();
+  });
+
   // ── Notes vs. itemized receipts (#1658) ────────────────────────────────────
 
   it('BUDGET-SVC-DB-021: a note and a receipt are stored in their own columns', () => {