feat(fusion_accounting_documents): Documents app <-> invoice bridge

Replaces Enterprise's documents_account with a Fusion-native bridge.
When a PDF/image lands in the Documents app, users can convert it
into a draft vendor bill via a wizard that copies the document's
binary onto the new account.move and posts a chatter note linking
back to the source document.

Adds:
- documents.document.move_id (Many2one to the linked invoice)
- documents.document.is_invoice_candidate (computed; True for
  unlinked PDF/image binaries)
- documents.document.action_create_invoice() opens the wizard
- account.move.source_document_ids reverse linkage + statinfo button
- fusion.create.invoice.from.document.wizard (TransientModel + form)
- ir.actions.server bound to documents.document so the workflow
  appears in the kanban/list Actions menu (the Documents app has
  no regular form view to inherit from in v19)

The wizard:
- defaults to the company's first purchase journal
- supports vendor bill or vendor credit note
- copies the source attachment onto the new move
- posts a chatter note linking back
- marks the document linked so it stops appearing as a candidate

Auto-installs when documents + fusion_accounting_core are both
present. 8 unit tests cover the candidate flag, wizard happy path,
attachment copy, reverse linkage, already-linked guard, non-PDF
guard, and credit-note creation.

Made-with: Cursor

fusion_accounting_documents/security/ir.model.access.csv

@@ -0,0 +1,2 @@
+id,name,model_id:id,group_id:id,perm_read,perm_write,perm_create,perm_unlink
+access_fusion_create_invoice_wizard_user,fusion.create.invoice.wizard.user,model_fusion_create_invoice_from_document_wizard,base.group_user,1,1,1,1