fix(monitoring): pg_archive_stalled must not false-fire on an idle database

The "WAL archiving stalled" alert fired on prod during a quiet period. An idle Postgres
archives nothing — it never force-switches an empty WAL segment on archive_timeout — so
pg_stat_archiver_last_archive_age grows past 30 min even though archiving is perfectly
healthy (failed_count 0, no .ready segments, pg_wal flat, backups current). A false
positive: no data and no disk at risk (nothing was written, so the frozen recovery point
equals the live state).

Gate the age condition on pg_wal actually growing:

  (pg_stat_archiver_last_archive_age > 1800) and on() (delta(pg_wal_size_bytes[35m]) > 16MB)

so it fires only when WAL is being produced but not archived (the real "pg_wal fills the
disk" danger; genuine archive_command failures are already caught by pg_archive_failing).
`and on()` bridges the two metrics' differing label sets (last_archive_age carries a server
label, the pg_wal gauge does not); delta() (not increase) suits the gauge. pg_stat_wal is
not exported by this postgres_exporter, so pg_wal size growth is the available signal.
Validated on prod with promtool: the expression is empty while idle.

Also fix the runbook command in both archive alerts' annotations: `pgbackrest check` needs
`--pg1-user=scrabble`, or it connects as role "root" (which does not exist) and aborts with
"no database found".

deploy/README.md

@@ -255,8 +255,10 @@ shipping or redeploying this stack does **not** start archiving — the artifact
 armed, which is why an un-armed prod deploy can never pile WAL onto the disk. The base-backup
 timer is provisioned by the Ansible `main` role behind `pitr_enabled` (also default off). Two
 Grafana alerts watch health: `WAL archiving failing` (`pg_stat_archiver_failed_count` rising)
-and `WAL archiving stalled` (`pg_stat_archiver_last_archive_age` over 30 min) — both
-absent/NaN-safe, so they stay quiet until archiving is armed.
+and `WAL archiving stalled` (last archive over 30 min old **while `pg_wal_size_bytes` is growing**
+— the pg_wal-growth guard keeps an idle database, which archives nothing because it writes nothing,
+from false-triggering during quiet hours) — both absent/NaN-safe, so they stay quiet until
+archiving is armed.
 
 **Assessment (owner-reviewed; the gate before the first real money).** Measured on prod
 `pg_stat_wal`: WAL is generated at **~0.77 MB/day** and the database is **~9.6 MB**. At