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).

scripts/backup.sh

@@ -2,8 +2,8 @@
 # Dumps the Siftlode Postgres database to backups/siftlode-<timestamp>.dump (custom format).
 # Run from the project root:  ./scripts/backup.sh
 set -e
-USER_NAME="${POSTGRES_USER:-subfeed}"
-DB_NAME="${POSTGRES_DB:-subfeed}"
+USER_NAME="${POSTGRES_USER:-siftlode}"
+DB_NAME="${POSTGRES_DB:-siftlode}"
 mkdir -p backups
 TS=$(date +%Y%m%d-%H%M%S)
 OUT="backups/siftlode-$TS.dump"