Add Render network passphrase config

[BojayL]

Summary

• Adds NETWORK_PASSPHRASE to the waveflow-gateway Render environment.
• Documents the testnet and mainnet Stellar network passphrases in the README and Render deployment guide.
• Adds startup validation for obvious testnet/mainnet RPC and passphrase mismatches.
• Keeps the environment variable reference in sync with the explicit testnet default.

Fixes #5.

Validation

• cargo test -p waveflow-shared
• rustfmt --check crates/shared/src/config.rs
• git diff --check
• ruby -e 'require "yaml"; YAML.load_file("render.yaml"); puts "render.yaml ok"'
• Checked PR-facing files for restricted terms.