fix(trips): preserve day content when trip date range changes

Rewrites generateDays to remap days positionally by day_number instead
of matching by date identity. Previously any date range shift with no
overlap would cascade-delete all day_assignments, day_notes, and
day_accommodations.

New behaviour:
- Shift/partial overlap: existing days remapped to new dates in order
- Shrink: overflow days become dateless (date=NULL) instead of deleted,
  preserving all child data for manual reassignment
- Grow: existing days kept, new empty days appended
- Clear dates: all days nullified, content intact

Also fixes a UNIQUE(trip_id, day_number) collision that would occur when
spare dateless days remained after growing into a partially-dateless trip
(maxAssigned base was wrong).

Closes #646

server/tests/integration/trips.test.ts

@@ -49,7 +49,7 @@ import { createApp } from '../../src/app';
 import { createTables } from '../../src/db/schema';
 import { runMigrations } from '../../src/db/migrations';
 import { resetTestDb } from '../helpers/test-db';
-import { createUser, createAdmin, createTrip, addTripMember, createPlace, createReservation, createTag, createDayAccommodation, createBudgetItem, createPackingItem, createDayNote } from '../helpers/factories';
+import { createUser, createAdmin, createTrip, addTripMember, createPlace, createReservation, createTag, createDayAccommodation, createBudgetItem, createPackingItem, createDayNote, createDayAssignment } from '../helpers/factories';
 import { authCookie } from '../helpers/auth';
 import { loginAttempts, mfaAttempts } from '../../src/routes/auth';
 import { invalidatePermissionsCache } from '../../src/services/permissions';
@@ -430,6 +430,65 @@ describe('Update trip', () => {
 
     expect(res.status).toBe(404);
   });
+
+  it('TRIP-023 — Shifting trip date range preserves day assignments positionally', async () => {
+    const { user } = createUser(testDb);
+    const trip = createTrip(testDb, user.id, { start_date: '2026-08-01', end_date: '2026-08-05' });
+
+    const days = testDb.prepare('SELECT * FROM days WHERE trip_id = ? ORDER BY day_number').all(trip.id) as { id: number; date: string }[];
+    expect(days).toHaveLength(5);
+
+    const place = createPlace(testDb, trip.id);
+    const assignment = createDayAssignment(testDb, days[0].id, place.id);
+    const note = createDayNote(testDb, days[1].id, trip.id, { text: 'pack sunscreen' });
+
+    // Shift forward 10 days (zero overlap with original range)
+    const res = await request(app)
+      .put(`/api/trips/${trip.id}`)
+      .set('Cookie', authCookie(user.id))
+      .send({ start_date: '2026-08-11', end_date: '2026-08-15' });
+
+    expect(res.status).toBe(200);
+
+    const daysAfter = testDb.prepare('SELECT * FROM days WHERE trip_id = ? ORDER BY day_number').all(trip.id) as { id: number; date: string | null }[];
+    expect(daysAfter).toHaveLength(5);
+    expect(daysAfter.map(d => d.date)).toEqual(['2026-08-11', '2026-08-12', '2026-08-13', '2026-08-14', '2026-08-15']);
+
+    const assignmentsAfter = testDb.prepare('SELECT * FROM day_assignments WHERE id = ?').get(assignment.id) as { day_id: number } | undefined;
+    expect(assignmentsAfter).toBeDefined();
+    expect(assignmentsAfter!.day_id).toBe(daysAfter[0].id);
+
+    const notesAfter = testDb.prepare('SELECT * FROM day_notes WHERE id = ?').get(note.id) as { day_id: number } | undefined;
+    expect(notesAfter).toBeDefined();
+    expect(notesAfter!.day_id).toBe(daysAfter[1].id);
+  });
+
+  it('TRIP-024 — Shrinking trip date range keeps overflow days as dateless with content intact', async () => {
+    const { user } = createUser(testDb);
+    const trip = createTrip(testDb, user.id, { start_date: '2026-09-01', end_date: '2026-09-05' });
+
+    const days = testDb.prepare('SELECT * FROM days WHERE trip_id = ? ORDER BY day_number').all(trip.id) as { id: number }[];
+    const place = createPlace(testDb, trip.id);
+    const a4 = createDayAssignment(testDb, days[3].id, place.id);
+    const a5 = createDayAssignment(testDb, days[4].id, place.id);
+
+    // Shrink from 5 to 3 days
+    const res = await request(app)
+      .put(`/api/trips/${trip.id}`)
+      .set('Cookie', authCookie(user.id))
+      .send({ start_date: '2026-09-01', end_date: '2026-09-03' });
+
+    expect(res.status).toBe(200);
+
+    const daysAfter = testDb.prepare('SELECT * FROM days WHERE trip_id = ? ORDER BY day_number').all(trip.id) as { id: number; date: string | null }[];
+    expect(daysAfter).toHaveLength(5);
+    expect(daysAfter.filter(d => d.date !== null)).toHaveLength(3);
+    expect(daysAfter.filter(d => d.date === null)).toHaveLength(2);
+
+    // Overflow assignments survived
+    const all = testDb.prepare('SELECT * FROM day_assignments WHERE id IN (?, ?)').all(a4.id, a5.id) as { id: number }[];
+    expect(all).toHaveLength(2);
+  });
 });
 
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