refactor: retire the images tarball backup

Images and attachments are backed up off-site, one object per file, so the
tar+zstd of /images goes: its service and restore, both tasks, the trigger and
restore routes, the Keep last setting, the screen's Images section, and the
tests. Migration 0122 drops backup_run.tar_path and
import_settings.backup_images_keep_last_n and deletes any images runs.

The local database dump stays for an install with no off-site destination,
and the Backups card now says plainly that images and attachments are backed
up only off-site.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

backend/app/models/import_settings.py

@@ -97,10 +97,6 @@ class ImportSettings(Base):
         Integer, nullable=False, default=14,
         server_default="14",
     )
-    backup_images_keep_last_n: Mapped[int] = mapped_column(
-        Integer, nullable=False, default=3,
-        server_default="3",
-    )
     # Off-site backup destination: an S3 bucket (services/backup_destination).
     # The secret key is stored sealed with the credential key, never in clear.
     backup_s3_endpoint: Mapped[str | None] = mapped_column(Text, nullable=True)