feat: implement core ERP modules, multi-tenant architecture, and comprehensive system workflow documentation.

Modules/ProjectManagement/app/Models/Project.php

@@ -110,6 +110,13 @@ class Project extends Model
         return $this->belongsTo(Contractor::class);
     }
 
+    public function contractors(): BelongsToMany
+    {
+        return $this->belongsToMany(Contractor::class, 'project_contractor')
+            ->withPivot('role', 'contract_amount')
+            ->withTimestamps();
+    }
+
     public function tasks(): HasMany
     {
         return $this->hasMany(Task::class)->orderBy('sort_order');