feat(fusion_accounting_followup): fusion.followup.engine 7-method API

The orchestrator AbstractModel for follow-up lifecycle.
get_overdue_for_partner, compute_followup_level, send_followup_email,
escalate_to_next_level, pause_followup, reset_followup, snapshot_followup_history.

All controllers, AI tools, wizards, cron must route through these
methods; no direct ORM writes to fusion.followup.run from anywhere else.

Made-with: Cursor

fusion_accounting_followup/models/__init__.py

@@ -3,3 +3,4 @@ from . import fusion_followup_run
 from . import fusion_followup_text_cache
 from . import res_partner
 from . import account_move_line
+from . import fusion_followup_engine