fix(fusion_accounting_followup): seeded levels + migration idempotency

- test_create_minimal/negative_delay used sequence=1, which now collides
  with the seeded Friendly Reminder level. Use sequences 901/902.
- migration backfill: search by name (not raw seq) for idempotency,
  allocate sequence as max(existing)+1 to avoid both seed clashes and
  within-batch collisions when Enterprise has duplicate sequence values.

Made-with: Cursor

fusion_accounting_followup/tests/test_fusion_followup_level.py

@@ -6,8 +6,9 @@ from odoo.tests import tagged
 class TestFusionFollowupLevel(TransactionCase):
 
     def test_create_minimal(self):
+        # Note: sequences 1-3 are reserved for seeded default levels.
         level = self.env['fusion.followup.level'].create({
-            'name': 'Reminder', 'sequence': 1, 'delay_days': 7, 'tone': 'gentle',
+            'name': 'Reminder', 'sequence': 901, 'delay_days': 7, 'tone': 'gentle',
         })
         self.assertEqual(level.name, 'Reminder')
         self.assertTrue(level.active)
@@ -15,7 +16,7 @@ class TestFusionFollowupLevel(TransactionCase):
     def test_negative_delay_rejected(self):
         with self.assertRaises(Exception):
             self.env['fusion.followup.level'].create({
-                'name': 'Bad', 'sequence': 1, 'delay_days': -5, 'tone': 'gentle',
+                'name': 'Bad', 'sequence': 902, 'delay_days': -5, 'tone': 'gentle',
             })
 
     def test_duplicate_sequence_rejected(self):