feat(deploy): continuous WAL archiving to S3 for point-in-time recovery

Add pgBackRest-based PITR for the production database, shipped disarmed
(archive_mode and the base-backup timer gated off) so it stays inert until the
operator arms it before real payments — an un-armed deploy cannot pile WAL onto
the disk.

- Postgres now builds from a thin image carrying pgBackRest (archive_command runs
  in-process); the prod overlay wires an encrypted (AES-256-CBC), path-style S3
  repository with 30-day retention and a daily full base-backup systemd timer.
- Repository config/secrets flow through the PROD_PGBACKREST_* Gitea set and
  write-prod-env.sh; prod-rollback re-renders the same env so a rollback cannot
  disarm archiving.
- Grafana alerts watch pg_stat_archiver (failing / stalled), absent-safe on the
  test contour, which never archives.
- Fix the pre-migration pg_dump to snapshot the whole database (was backend-only,
  silently excluding payments).
- Document the PITR runbook, the arming sequence and the restore drill in
  deploy/README.md; record the measured cost/perf assessment.

deploy/docker-compose.yml

@@ -38,7 +38,13 @@ x-logging: &default-logging
 services:
   postgres:
     container_name: scrabble-postgres
-    image: postgres:17-alpine
+    # Built from postgres:17-alpine + pgBackRest (deploy/postgres/Dockerfile) so the prod
+    # main host can archive WAL for point-in-time recovery. Archiving is switched on only by
+    # the prod overlay (docker-compose.prod.yml); on this contour the extra binary is present
+    # but idle and this is a plain postgres. See deploy/README.md (point-in-time recovery).
+    image: scrabble-postgres:latest
+    build:
+      context: postgres
     restart: unless-stopped
     logging: *default-logging
     environment: