feat(fusion_accounting_bank_rec): migration wizard bootstrap step

Adds bank_rec_bootstrap step that backfills fusion.reconcile.precedent
from existing account.partial.reconcile rows during migration. This
gives the AI memory from past Enterprise reconciles. Also triggers
pattern refresh + MV refresh for immediate UI readiness.

- New service services/precedent_backfill.py walks
  account.partial.reconcile rows, identifies the bank-statement-line
  side, and creates a precedent per qualifying partial. Idempotent via
  (statement_line, account, amount, source='backfill') signature.
- New model models/fusion_migration_wizard.py inherits
  fusion.migration.wizard, exposes _bank_rec_bootstrap_step() (callable
  from tests/audit), and overrides action_run_migration() to call
  super() + the bootstrap.
- Adds 'backfill' to fusion.reconcile.precedent.source selection.
- Adds fusion_accounting_migration to depends.

Made-with: Cursor

fusion_accounting_bank_rec/__manifest__.py

@@ -1,6 +1,6 @@
 {
     'name': 'Fusion Accounting — Bank Reconciliation',
-    'version': '19.0.1.0.20',
+    'version': '19.0.1.0.21',
     'category': 'Accounting/Accounting',
     'sequence': 28,
     'summary': 'Native V19 bank reconciliation widget with AI confidence scoring + behavioural learning.',
@@ -24,7 +24,7 @@ Built by Nexa Systems Inc.
     'author': 'Nexa Systems Inc.',
     'website': 'https://nexasystems.ca',
     'maintainer': 'Nexa Systems Inc.',
-    'depends': ['fusion_accounting_core'],
+    'depends': ['fusion_accounting_core', 'fusion_accounting_migration'],
     'external_dependencies': {
         'python': ['hypothesis'],
     },