[codex] Add reproducible local backend startup

[SH-ZMD]

What changed

• add a multi-stage JDK 25 Docker image and Docker Compose development stack
• provision PostgreSQL and Redis with health-based startup ordering and persistent development volumes
• use a verified regional Gradle mirror first with official repositories retained as fallback
• add Gradle cache and bounded retries for transient dependency-download failures
• disable the nonessential mail health indicator only in the local Docker environment
• document Docker-first startup and retain the existing manual setup path

Why

A fresh clone could not start without manually installing JDK 25, PostgreSQL, Redis, and a complete .env. Container builds could also fail on Gradle Plugin Portal TLS interruptions, while an unconfigured SMTP server made an otherwise healthy local stack report 503.

Validation

• docker compose config: passed
• clean Docker bootJar build with JDK 25: passed
• PostgreSQL and Redis health checks: passed
• backend Actuator health: overall UP with database and Redis UP
• frontend-to-backend CORS request: passed with HTTP 200
• real browser smoke test against the Docker backend: passed