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).

docker-compose.selfhost.yml

@@ -14,16 +14,16 @@ services:
   db:
     image: postgres:16-alpine
     environment:
-      POSTGRES_USER: ${POSTGRES_USER:-subfeed}
+      POSTGRES_USER: ${POSTGRES_USER:-siftlode}
       POSTGRES_PASSWORD: ${POSTGRES_PASSWORD:?POSTGRES_PASSWORD must be set (run install.sh)}
-      POSTGRES_DB: ${POSTGRES_DB:-subfeed}
+      POSTGRES_DB: ${POSTGRES_DB:-siftlode}
     volumes:
       - siftlode_pgdata:/var/lib/postgresql/data
     networks: [internal]
     security_opt:
       - no-new-privileges:true
     healthcheck:
-      test: ["CMD-SHELL", "pg_isready -U ${POSTGRES_USER:-subfeed} -d ${POSTGRES_DB:-subfeed}"]
+      test: ["CMD-SHELL", "pg_isready -U ${POSTGRES_USER:-siftlode} -d ${POSTGRES_DB:-siftlode}"]
       interval: 10s
       timeout: 5s
       retries: 12
@@ -34,7 +34,7 @@ services:
     env_file:
       - .env
     environment:
-      DATABASE_URL: postgresql+psycopg://${POSTGRES_USER:-subfeed}:${POSTGRES_PASSWORD}@db:5432/${POSTGRES_DB:-subfeed}
+      DATABASE_URL: postgresql+psycopg://${POSTGRES_USER:-siftlode}:${POSTGRES_PASSWORD}@db:5432/${POSTGRES_DB:-siftlode}
       # This instance owns the background scheduler (single writer).
       SCHEDULER_ENABLED: "true"
     depends_on: