Add STELLAR_NETWORK configuration validation

[meshackyaro]

Description

STELLAR_NETWORK accepts unknown values silently defaulting to testnet. Dangerous misconfiguration possible.

Component

Backend

Difficulty

🟢 Easy

Tasks

• Validate STELLAR_NETWORK enum values
• Fail fast on unknown values
• Document accepted values (TESTNET/PUBLIC)
• Update .env.example
• Test with invalid values

Acceptance Criteria

• Unknown networks rejected at startup
• Clear error message
• Valid values documented

Estimated Time

1 day