feat(ops-api/site-mappings): provision monitoring DB pool and readiness check

Open a separate pgx pool for the monitoring database, wire it into the
SiteDomainMappingService, and surface its health in /readyz. Point compose
at the dedicated monitoring-postgres service and align local/dev configs
with the per-instance ports. Also reformat the site_domain_mapping types
with gofmt while the files are touched.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

deploy/docker-compose.yaml

@@ -240,6 +240,8 @@ services:
     depends_on:
       postgres:
         condition: service_healthy
+      monitoring-postgres:
+        condition: service_healthy
       migrate:
         condition: service_completed_successfully
     ports: