feat(fusion_accounting_ai): wire FollowupAdapter fusion paths to engine

- Switch FUSION_MODEL to fusion.followup.engine so adapter mode
  selection matches the new module
- Add list_overdue() with fusion/enterprise/community variants
- Re-route send_followup_via_fusion to engine.send_followup_email
- 4 new TransactionCase tests (73 total)

Existing aging / overdue_invoices adapter methods continue to fall
back to the community implementation.

Made-with: Cursor

fusion_accounting_followup/__manifest__.py

@@ -1,6 +1,6 @@
 {
     'name': 'Fusion Accounting Follow-up',
-    'version': '19.0.1.0.15',
+    'version': '19.0.1.0.16',
     'category': 'Accounting/Accounting',
     'summary': 'AI-augmented customer follow-ups (dunning) for unpaid invoices.',
     'description': """