fix(config): require secure WEBHOOK_SECRET configuration

[whiteghost0001]

Summary
This pull request addresses the security vulnerability where WEBHOOK_SECRET defaulted to a guessable, publicly-known literal value ("default-secret"). It removes the default fallback and implements a strict configuration validation flow at server startup. Additionally, this PR fixes a pre-existing compilation error in the payment rate limiter module.

Changes

1. Removed Default Webhook Secret Fallback: Deleted the env_or helper function in src/config.rs and replaced it with explicit environment variables checks to prevent silently falling back to insecure or guessable defaults for sensitive configurations.
2. Added Configuration Validation at Boot: Introduced the validate_webhook_secret helper to validate WEBHOOK_SECRET in Config::from_env(). Validation fails fast at boot if the secret:
   • Is missing/not set in the environment.
   • Is empty.
   • Contains only whitespace.
   • Equals the old guessable default ("default-secret").
   • Is shorter than 32 characters.
3. Preserved Out-of-the-Box Local Dev Experience: Updated .env.example with a dummy value of 32+ characters (default-development-webhook-signing-secret-32-chars) so that developers copying the example template have a valid config out-of-the-box.
4. Cleaned Up Rate-Limiter Compilation Issue: Removed a redundant and broken inner rate-limit middleware layer in src/api/mod.rs that referred to an undefined variable (rate_limit_rps) and a non-existent module (tower_http::util).

Security Impact
Enforcing strict, fail-fast validation at boot prevents server instances from running without a configured webhook secret or with weak, guessable secrets, closing a vulnerability that could allow attackers to forge webhook signatures.

Testing Performed

• Appended a comprehensive suite of unit tests to the end of the tests module in src/config.rs, validating each validation constraint (missing, empty, whitespace, guessable default, short secret, valid long secret, production startup failure, and successful startup).
• Verified that all unit, integration, and doc tests pass cleanly (cargo test).
• Verified code quality and styling check out without issues (cargo fmt -- --check and cargo clippy --all-targets --all-features -- -D warnings).

closes #68