fix: load and validate required env vars at startup (#24)

[Chucks1093]

Closes #24

Changes

• Added dotenvy crate to load environment variables from .env at startup.
• Added a new config module at src/config.rs with:
  • Config struct
  • Config::from_env() loader
  • Required variable checks for:
    • STELLAR_NETWORK
    • STELLAR_PLATFORM_SECRET
    • HORIZON_URL
    • SOROBAN_RPC_URL
  • Descriptive errors for missing and empty required variables.
• Updated startup flow in src/main.rs to:
  • load and validate config at process start
  • print Startup configuration error: ... and exit with code 1 if invalid
• Updated .env.example with all required keys and clear descriptions.
• Updated Cargo.lock for new dependency resolution.

Testing

• Ran cargo test in repository root.
• Result: all tests passing (41 passed, 0 failed).

Acceptance Criteria Check

• Service now fails at startup with a clear message when any required env var is missing.
• Required env vars are loaded and validated at startup.
• .env.example includes all required keys with descriptions.