Fix postgres healthcheck probing a nonexistent database

pg_isready without -d connects to a database named after the user, but
the database is imapcopier, so every probe logged a FATAL and the
healthcheck only passed because pg_isready treats "server rejects the
connection" as reachable.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

docker-compose.yml

@@ -8,7 +8,7 @@ services:
     volumes:
       - pgdata:/var/lib/postgresql
     healthcheck:
-      test: ["CMD-SHELL", "pg_isready -U imap imapcopier"]
+      test: ["CMD-SHELL", "pg_isready -U imap -d imapcopier"]
       interval: 5s
       timeout: 3s
       retries: 5