feat(preprod): Add snapshot comparison Celery task

Add compare_snapshots task that loads manifests from objectstore,
diffs matched images in pixel-budget batches via OdiffServer, uploads
diff masks, and writes comparison results with state machine guards.

Author: NicoHinderling

src/sentry/conf/server.py

@@ -887,6 +887,7 @@ def SOCIAL_AUTH_DEFAULT_USERNAME() -> str:
     "sentry.notifications.platform.service",
     "sentry.notifications.utils.tasks",
     "sentry.preprod.size_analysis.tasks",
+    "sentry.preprod.snapshots.tasks",
     "sentry.preprod.tasks",
     "sentry.preprod.vcs.status_checks.size.tasks",
     "sentry.profiles.task",