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/models/account_move.py

@@ -0,0 +1,33 @@
+"""Reverse linkage from account.move back to source documents."""
+
+from odoo import _, fields, models
+
+
+class AccountMove(models.Model):
+    _inherit = 'account.move'
+
+    source_document_ids = fields.One2many(
+        'documents.document',
+        'move_id',
+        string='Source Documents',
+        readonly=True,
+        help="Documents in the Documents app that were used to create this move.",
+    )
+    source_document_count = fields.Integer(
+        string='Source Document Count',
+        compute='_compute_source_document_count',
+    )
+
+    def _compute_source_document_count(self):
+        for m in self:
+            m.source_document_count = len(m.source_document_ids)
+
+    def action_open_source_documents(self):
+        self.ensure_one()
+        return {
+            'type': 'ir.actions.act_window',
+            'name': _('Source Documents'),
+            'res_model': 'documents.document',
+            'view_mode': 'kanban,list',
+            'domain': [('move_id', '=', self.id)],
+        }