fix(docker): harden Postgres readiness checks in compose and startup (#595)

Pin the compose Postgres healthchecks to POSTGRES_DB (pg_isready was
defaulting to the username, silently reporting healthy while spamming
FATAL logs when USER and DB differ), and make docker/wait-for-postgres.mjs
log the target host and error code instead of a silent retry loop. Adds a
change-me note next to the default password in README and Docker Hub.

Refs #592

DOCKERHUB.md

@@ -58,6 +58,7 @@ services:
     image: postgres:17-alpine
     environment:
       POSTGRES_USER: snapotter
+      # Change this for any non-local deployment.
       POSTGRES_PASSWORD: snapotter
       POSTGRES_DB: snapotter
     volumes: ["SnapOtter-pgdata:/var/lib/postgresql/data"]