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/scheduler.py

@@ -27,7 +27,7 @@ from app.sync.runner import (
     run_subscription_resync,
 )
 
-logger = logging.getLogger("subfeed.scheduler")
+logger = logging.getLogger("siftlode.scheduler")
 
 _scheduler: BackgroundScheduler | None = None