fix: derive contingent change signal from the sync diff

src/Entity/Groups/ContingentSyncState.php

@@ -11,8 +11,8 @@ use Doctrine\ORM\Mapping as ORM;
 /**
  * Bookkeeping for the scheduled contingent sync of a single accommodation.
  *
- * Holds the fingerprint of the stored contingent days, which is how the sync decides whether
- * anything actually changed, and `changedAt` records when it last did.
+ * `changedAt` records when the sync last saw a day appear, vanish or switch status within the
+ * horizon it had already reached; `horizonTo` is how far that reach extends.
  */
 #[ORM\Entity(repositoryClass: ContingentSyncStateRepository::class)]
 #[ORM\Table(name: 'contingent_sync_state')]
@@ -28,13 +28,7 @@ class ContingentSyncState
     private ?Accommodation $accommodation = null;
 
     /**
-     * sha256 over the stored contingent days, contingent status only.
-     */
-    #[ORM\Column(length: 64)]
-    private string $contentHash = '';
-
-    /**
-     * Last time the fingerprint actually changed. Kept for operators, not exposed via the API.
+     * Last time the stored snapshot actually changed. Kept for operators, not exposed via the API.
      */
     #[ORM\Column(type: Types::DATETIME_IMMUTABLE, nullable: true)]
     private ?\DateTimeImmutable $changedAt = null;
@@ -76,18 +70,6 @@ class ContingentSyncState
         return $this;
     }
 
-    public function getContentHash(): string
-    {
-        return $this->contentHash;
-    }
-
-    public function setContentHash(string $contentHash): self
-    {
-        $this->contentHash = $contentHash;
-
-        return $this;
-    }
-
     public function getChangedAt(): ?\DateTimeImmutable
     {
         return $this->changedAt;