feat: add sous-devis generation from commandes

From a commande, users can now create one or more sub-quotes
(sous-devis) to ventilate purchases across communes for approval.

- Add parent_devis_id and commande_id FKs on devis table
- Order model: sousDevis() hasMany relation
- Devis model: commande() and parent() relations
- DevisController: createFromCommande / storeFromCommande methods
- OrderController: load and pass sous_devis on show
- Commandes/Show: "Créer un sous-devis" button + ventilation panel
- Devis/CreateFromCommande: form pre-loaded with commande context
- Devis/Index: badge "Sous-devis · CMD-XXXX" on linked quotes
- Devis/Show: back-link to originating commande

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/Models/Order.php

@@ -45,6 +45,11 @@ class Order extends Model
         return $this->hasMany(Attachment::class);
     }
 
+    public function sousDevis()
+    {
+        return $this->hasMany(\App\Models\Devis::class, 'commande_id')->latest();
+    }
+
     /**
      * Scope par statut(s).
      */