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/Http/Controllers/OrderController.php

@@ -194,10 +194,18 @@ class OrderController extends Controller
     {
         Gate::authorize('view', $order);
 
-        $order->load(['attachments', 'statusLogs.user']);
+        $order->load(['attachments', 'statusLogs.user', 'sousDevis']);
 
         return Inertia::render('Commandes/Show', [
-            'order' => new OrderResource($order),
+            'order'     => new OrderResource($order),
+            'sousDevis' => $order->sousDevis->map(fn($d) => [
+                'id'               => $d->id,
+                'number'           => $d->number,
+                'title'            => $d->title,
+                'destinataire_nom' => $d->destinataire_nom,
+                'total_ttc'        => (float) $d->total_ttc,
+                'status'           => $d->status,
+            ]),
         ]);
     }