feat(fusion_accounting_ai): add post-migration to reassign ir_model_data ownership

Phase 0 Task 7. Pre-Phase-0 all AI code lived in module='fusion_accounting';
the code now lives in 'fusion_accounting_ai' but existing ir_model_data
rows still record the old module name. This post-migration rewrites them.

Handles duplicate-key conflicts by deleting old orphan rows when data-load
has already created a new row under the same name in the new module.

Idempotent: second run reassigns 0 rows.
Made-with: Cursor

fusion_accounting_ai/tests/__init__.py

@@ -0,0 +1 @@
+from . import test_post_migration