chore: rename remaining subfeed references to siftlode

Replace the leftover 'subfeed' name across logger names + log_config,
frontend localStorage keys, Postgres user/db/volume defaults in the
compose files, .env.example, config.py, backup/restore scripts and the
README. Pure rename; no behavioural change. localStorage keys move from
subfeed.* to siftlode.* (one-time UI-state reset is acceptable).

backend/app/sync/maintenance.py

@@ -34,7 +34,7 @@ from app.sync.runner import get_service_user
 from app.sync.videos import apply_video_details, parse_dt
 from app.youtube.client import YouTubeClient
 
-log = logging.getLogger("subfeed.maintenance")
+log = logging.getLogger("siftlode.maintenance")
 
 # Reasons recorded on Video.unavailable_reason. "removed" = absent from videos.list
 # (deleted or made private); "abandoned" = an upcoming premiere that never went live.