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/log_config.json

@@ -35,6 +35,6 @@
     "uvicorn": { "handlers": ["default"], "level": "INFO", "propagate": false },
     "uvicorn.error": { "handlers": ["default"], "level": "INFO", "propagate": false },
     "uvicorn.access": { "handlers": ["access"], "level": "INFO", "propagate": false },
-    "subfeed": { "handlers": ["plain"], "level": "INFO", "propagate": false }
+    "siftlode": { "handlers": ["plain"], "level": "INFO", "propagate": false }
   }
 }