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/models/fusion_reconcile_precedent.py

@@ -41,6 +41,7 @@ class FusionReconcilePrecedent(models.Model):
     reconciled_at = fields.Datetime()
     source = fields.Selection([
         ('historical_bootstrap', 'Imported from history'),
+        ('backfill', 'Backfilled from account.partial.reconcile (migration)'),
         ('manual', 'Manual reconcile via fusion'),
         ('ai_accepted', 'AI suggestion accepted'),
         ('auto_rule', 'account.reconcile.model auto-fired'),