fix: the images backup carried the key to the accounts it backs up (4234)

Listing a 2026-05 tarball while investigating the 4.3T `_backups` pile showed
its second and third entries:

    images/secrets/
    images/secrets/credential_key.b64

That is the key that decrypts the stored Patreon/SubscribeStar session
credentials, and `cookies/` sat beside it — both unexcluded, so this was true
of every images backup taken today, not just the old ones. An images tarball
is supposed to be a media archive; one that carries the operator's account
keys is a credential leak wearing a backup's name, in a single file that is
easy to copy to another disk or restore somewhere less protected. Encryption
at rest buys nothing when the key travels in the same archive.

`secrets` and `cookies` join `_backups` and `_quarantine` in one named tuple,
each with its reason recorded — the recursion that produced 4.3T of nested
tarballs is the cautionary tale for why the list is worth explaining rather
than just listing.

A restore no longer re-establishes credentials. You sign in again, which is
the correct outcome for a media backup.

Tests cover both new names and that every exclude stays root-relative — a bare
`secrets` would also match an artist folder of that name.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LVjrnpQjRgHdvq95rASoiR

backend/app/services/backup_service.py

@@ -24,6 +24,25 @@ from pathlib import Path
 
 _BACKUPS_DIRNAME = "_backups"
 
+# Excluded from the images tarball, and each for its own reason (#4233, #4234):
+#
+#   _backups    — the archive would otherwise contain every previous archive.
+#                 This is not hypothetical: the 2026-05-23/24 runs, taken before
+#                 this exclude existed, grew 43G -> 107G -> ... -> 2123G as each
+#                 swallowed its predecessors, and cost 4.3T of the images
+#                 filesystem until they were reclaimed on 2026-09-21.
+#   _quarantine — holds files deliberately pulled OUT of the library.
+#   secrets     — `credential_key.b64`, the key that decrypts the stored
+#                 Patreon/SubscribeStar session credentials.
+#   cookies     — those session cookies themselves.
+#
+# The last two are the ones worth stating plainly: an images tarball is a media
+# archive, and a media archive that carries the key to the operator's accounts
+# is a credential leak wearing a backup's name. Encryption at rest buys nothing
+# when the key rides along in the same file. A restore therefore does NOT
+# re-establish credentials — you sign in again, which is the correct outcome.
+_IMAGES_EXCLUDED_DIRNAMES = ("_backups", "_quarantine", "secrets", "cookies")
+
 # Subprocess-level guardrails BEYOND the Celery soft_time_limit. The Celery
 # soft limit signals the Python process; subprocess.Popen in a blocking syscall
 # ignores that signal, so these bound the worst case directly. Each sits just
@@ -173,8 +192,10 @@ def backup_images(
         [
             "tar", "--zstd", "-cf", str(tar_path),
             "-C", str(images_root.parent), images_root.name,
-            f"--exclude={images_root.name}/_backups",
-            f"--exclude={images_root.name}/_quarantine",
+            *(
+                f"--exclude={images_root.name}/{name}"
+                for name in _IMAGES_EXCLUDED_DIRNAMES
+            ),
         ],
         _IMAGES_SUBPROCESS_TIMEOUT_S,
     )