Configuration Management & Validation on Startup

[mftee]

Implement a centralized AppConfig struct that loads, validates, and provides all application configuration from environment variables at startup. The service should fail fast with clear error messages if required configuration is missing or invalid.

Requirements:

• Create an AppConfig struct in config.rs:

  pub struct AppConfig {
      pub port: u16,
      pub stellar_horizon_url: String,
      pub stellar_secret_key: Option<String>,
      pub redis_url: String,
      pub rate_limit_per_second: u32,
      pub rate_limit_burst: u32,
      pub stellar_max_retries: u32,
      pub log_level: String,
      pub webhook_urls: Vec<String>,
      pub webhook_secret: Option<String>,
      pub cache_verification_ttl: u64,
  }

• Implement AppConfig::from_env() -> Result<Self, ConfigError> that:
  • Reads all values from environment variables
  • Applies defaults for optional values
  • Validates: stellar_horizon_url is a valid URL, port is in range 1–65535, rate_limit_per_second > 0
  • Returns a descriptive ConfigError listing ALL validation failures at once (not just the first one)
• Replace all std::env::var calls scattered in main.rs and other modules with fields from AppConfig
• Print a startup summary of all resolved configuration values (redacting secrets) at INFO level

Acceptance Criteria:

• Starting with no env vars set uses all defaults and starts successfully
• Setting an invalid URL for STELLAR_HORIZON_URL causes a startup failure with a clear message
• Multiple validation errors are all reported together
• Secrets (stellar_secret_key, webhook_secret) are redacted in startup logs (shown as [REDACTED])
• Unit tests for from_env() covering valid config, missing required fields, and invalid values