setup: add docker-compose, Dockerfile, and .env.example

[Lewiskunta]

Implements - local development environment.

Files added:

• docker-compose.yml: starts Ratify app + PostgreSQL with one command
• docker-compose.dev.yml: development override with Air live reload
• docker/Dockerfile: two-stage production build, final image under 50MB
• docker/Dockerfile.dev: development image with Air for live reload
• .air.toml: Air configuration for Go live reload
• .env.example: environment variable template with safe local defaults
• templates/.gitkeep: placeholder for email templates directory

After this PR, contributors can run:
cp .env.example .env
docker compose up

And get a running Ratify instance with a healthy PostgreSQL database.