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/email.py

@@ -14,7 +14,7 @@ from app import sysconfig
 from app.config import settings
 from app.db import SessionLocal
 
-log = logging.getLogger("subfeed.email")
+log = logging.getLogger("siftlode.email")
 
 
 def _smtp() -> dict: