Postgres WAL on the NVMe; wal_buffers 64 MB

Every commit waited on the WAL in ./pg on the HDD (WALWrite; the HDD 100%
busy, 56–70 ms a write): prober flushes and claims took most of each minute.
A PG_WAL_DIR volume (default /var/lib/usenet-rebuild-wal) at
/var/lib/postgresql/wal, with data/pg_wal a symlink to it (set up by hand
once, with Postgres stopped).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

deploy/compose.yaml

@@ -42,6 +42,9 @@ services:
       # nvme, the database's default_tablespace (LOG.md 2026-10-09 (14)).
       - ./pg:/var/lib/postgresql/data
       - ${PG_NVME_DIR:-/var/lib/usenet-rebuild-catalog}:/var/lib/postgresql/nvme
+      # The WAL on the NVMe too: data/pg_wal is a symlink to /var/lib/postgresql/wal. On the HDD every commit
+      # waited on it (WALWrite, 56–70 ms a write with the HDD 100% busy; LOG.md 2026-10-13 (43)).
+      - ${PG_WAL_DIR:-/var/lib/usenet-rebuild-wal}:/var/lib/postgresql/wal
     # synchronous_commit = off is set with ALTER SYSTEM (postgresql.auto.conf in the data dir).
     command:
       - postgres
@@ -53,6 +56,8 @@ services:
       - max_wal_size=8GB
       - -c
       - checkpoint_timeout=15min
+      - -c
+      - wal_buffers=64MB
     shm_size: 1g
     stop_grace_period: 120s
     healthcheck: