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/Devis.php

@@ -3,6 +3,8 @@
 namespace App\Models;
 
 use Illuminate\Database\Eloquent\Model;
+use Illuminate\Database\Eloquent\Relations\BelongsTo;
+use Illuminate\Database\Eloquent\Relations\HasMany;
 
 class Devis extends Model
 {
@@ -16,6 +18,21 @@ class Devis extends Model
         'tva_rate' => 'decimal:2',
     ];
 
+    public function parent(): BelongsTo
+    {
+        return $this->belongsTo(Devis::class, 'parent_devis_id');
+    }
+
+    public function sousDevis(): HasMany
+    {
+        return $this->hasMany(Devis::class, 'parent_devis_id');
+    }
+
+    public function commande(): BelongsTo
+    {
+        return $this->belongsTo(\App\Models\Order::class, 'commande_id');
+    }
+
     protected static function booted()
     {
         static::creating(function ($devis) {