fix: reduce snapshot payloads via serializer group and hydration

src/Entity/TravelSnapshot.php

@@ -62,8 +62,8 @@ class TravelSnapshot
     #[ORM\Column(type: 'date_immutable', nullable: true)]
     private ?\DateTimeImmutable $dateTo = null;
 
-    /** Full JSON-serialized Travel aggregate. */
-    #[ORM\Column(type: 'text')]
+    /** Full serialized Travel aggregate stored as JSON text. */
+    #[ORM\Column(type: 'text', columnDefinition: 'LONGTEXT NOT NULL')]
     private string $payload;
 
     /** SHA-256 of payload; compared before every write to skip no-op flushes. */