user affiliation schema completed

backend/app/Models/Institution.php

@@ -87,4 +87,14 @@ class Institution extends Model implements Auditable
     {
         return $this->hasMany(InstitutionLink::class);
     }
+
+    /**
+     * Storico delle affiliazioni (staff) di questa istituzione.
+     *
+     * @return HasMany<UserAffiliation, $this>
+     */
+    public function affiliations(): HasMany
+    {
+        return $this->hasMany(UserAffiliation::class);
+    }
 }