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.server.yml

@@ -10,7 +10,7 @@
 #   export COMPOSE_FILE=docker-compose.server.yml   # or put it in .env
 #   docker compose up -d --build
 #
-# Postgres data lives in ./pgdata (a host-visible bind mount under /docker/subfeed)
+# Postgres data lives in ./pgdata (a host-visible bind mount under /docker/siftlode)
 # so it is covered by the host's /docker backups; logical pg_dump backups via
 # scripts/backup.sh remain the portable mechanism for moving between hosts.
 
@@ -18,9 +18,9 @@ services:
   db:
     image: postgres:16-alpine
     environment:
-      POSTGRES_USER: ${POSTGRES_USER:-subfeed}
-      POSTGRES_PASSWORD: ${POSTGRES_PASSWORD:-subfeed}
-      POSTGRES_DB: ${POSTGRES_DB:-subfeed}
+      POSTGRES_USER: ${POSTGRES_USER:-siftlode}
+      POSTGRES_PASSWORD: ${POSTGRES_PASSWORD:-siftlode}
+      POSTGRES_DB: ${POSTGRES_DB:-siftlode}
     volumes:
       - ./pgdata:/var/lib/postgresql/data
     ports:
@@ -31,7 +31,7 @@ services:
     security_opt:
       - apparmor:unconfined
     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: 5s
       timeout: 5s
       retries: 12
@@ -47,7 +47,7 @@ services:
         BUILD_DATE: ${BUILD_DATE:-}
     env_file: .env
     environment:
-      DATABASE_URL: postgresql+psycopg://${POSTGRES_USER:-subfeed}:${POSTGRES_PASSWORD:-subfeed}@db:5432/${POSTGRES_DB:-subfeed}
+      DATABASE_URL: postgresql+psycopg://${POSTGRES_USER:-siftlode}:${POSTGRES_PASSWORD:-siftlode}@db:5432/${POSTGRES_DB:-siftlode}
       # This instance owns the background sync. Keep it true here and false everywhere else.
       SCHEDULER_ENABLED: "true"
     depends_on: