feat: images and attachments go to the off-site bucket once each, by fingerprint

With an off-site destination set up, the library's files are sent to the
bucket's media/ folder as media/ab/cd/<sha256><ext>, beside the database
dumps under database/. Each file goes once: a new backup_blob table records
what each destination holds (migration 0121), and a run's work list is every
image and attachment sha with no row yet, so runs are incremental and a new
bucket starts its copy from the beginning.

- The bucket checks each file against the sha recorded at import; a file
  over 1 GiB goes in parts and is checked before the upload is completed.
  A file missing or changed on disk is reported, not stored.
- A run is chunked: each 45-minute chunk commits after every file and queues
  the next, so other long-maintenance jobs get turns. One run at a time.
  The stall sweep judges a media run by its progress heartbeat
  (backup_run.last_progress_at), not its start.
- A file that can't be sent is listed on the run (status ok, with the error
  line) and tried again by the next run. A refused put for an object
  already in an append-only bucket counts as sent.
- Nightly, with a destination set, runs the dump and then the media.
- Weekly reconcile lists media/ and re-sends what the bucket lost, and
  adopts what it already holds (a restored database sends nothing twice).
- Uploads use the bandwidth limit; read transactions end before transfers.
- Backups card: with a destination, an "Images and attachments" section
  shows coverage (files and bytes in the bucket), "Send new files now", and
  the media runs. Without one, the local tarball section is unchanged.
- Media run history keeps the latest 60 rows.

Task #6879, milestone 586.

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

backend/app/models/__init__.py

@@ -4,6 +4,7 @@ from .app_setting import AppSetting
 from .artist import Artist
 from .artist_membership_suggestion import ArtistMembershipSuggestion
 from .artist_visit import ArtistVisit
+from .backup_blob import BackupBlob
 from .backup_run import BackupRun
 from .base import Base
 from .character_prototype import CcipPrototypeState, CharacterPrototype
@@ -55,6 +56,7 @@ __all__ = [
     "Artist",
     "ArtistMembershipSuggestion",
     "ArtistVisit",
+    "BackupBlob",
     "BackupRun",
     "Source",
     "Credential",