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>

database/migrations/2026_06_19_080000_add_parent_devis_id_to_devis_table.php

@@ -0,0 +1,27 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    public function up(): void
+    {
+        Schema::table('devis', function (Blueprint $table) {
+            $table->foreignId('parent_devis_id')
+                ->nullable()
+                ->after('id')
+                ->constrained('devis')
+                ->nullOnDelete();
+        });
+    }
+
+    public function down(): void
+    {
+        Schema::table('devis', function (Blueprint $table) {
+            $table->dropForeignIdFor(\App\Models\Devis::class, 'parent_devis_id');
+            $table->dropColumn('parent_devis_id');
+        });
+    }
+};

database/migrations/2026_06_19_090000_add_commande_id_to_devis_table.php

@@ -0,0 +1,28 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    public function up(): void
+    {
+        Schema::table('devis', function (Blueprint $table) {
+            $table->foreignId('commande_id')
+                ->nullable()
+                ->after('parent_devis_id')
+                ->constrained('orders')
+                ->nullOnDelete();
+            $table->string('commande_number')->nullable()->after('commande_id');
+        });
+    }
+
+    public function down(): void
+    {
+        Schema::table('devis', function (Blueprint $table) {
+            $table->dropForeignIdFor(\App\Models\Order::class, 'commande_id');
+            $table->dropColumn(['commande_id', 'commande_number']);
+        });
+    }
+};