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/config.py

@@ -30,7 +30,7 @@ class Settings(BaseSettings):
     git_sha: str = "unknown"
     build_date: str = ""
 
-    database_url: str = "postgresql+psycopg://subfeed:subfeed@db:5432/subfeed"
+    database_url: str = "postgresql+psycopg://siftlode:siftlode@db:5432/siftlode"
 
     # Session cookie signing key.
     secret_key: str = _DEFAULT_SECRET_KEY