Update README to reflect current CLI flags, Postgres, and S3 support

[andrew]

The README still referenced old flag names and was missing documentation for features that have since been added.

• Update Go version requirement from 1.23 to 1.25
• Fix CLI flags to match actual names (-storage-url, -database-driver, -database-path, -database-url)
• Add missing environment variables (PROXY_STORAGE_URL, PROXY_DATABASE_DRIVER, PROXY_DATABASE_URL)
• Update config file example to use storage.url and database.driver
• Add PostgreSQL documentation section
• Add S3/S3-compatible storage documentation section
• Replace inline Dockerfile with reference to the real one, add Postgres/S3 docker example
• Update stats command examples with current flag names