fix(deps): pin gallery-dl>=1.31.10 to resolve Patreon 426

Earlier container rolled 1.31.5, which fails Patreon fetches with
HTTP 426 Upgrade Required. 1.31.10 restores the OAuth flow.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/requirements.txt

@@ -20,8 +20,8 @@ python-dotenv
 cryptography
 pyyaml
 
-# gallery-dl
-gallery-dl
+# gallery-dl (>=1.31.10 fixes Patreon HTTP 426 / OAuth flow)
+gallery-dl>=1.31.10
 
 # Testing
 pytest