Add healthcheck to postgresql17 (pg_isready)

Tier-1 health visibility for shared Postgres cluster. pg_isready
against the admin DB; 30s interval, 30s start_period.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

infra/postgresql17/docker-compose.yml

@@ -21,6 +21,13 @@ services:
     security_opt:
       - no-new-privileges:true
 
+    healthcheck:
+      test: ["CMD-SHELL", "pg_isready -U mailarchiver -d mailarchiver"]
+      interval: 30s
+      timeout: 5s
+      retries: 5
+      start_period: 30s
+
 networks:
   backend_net:
     external: true