feat(cors): implement CORS configuration with From trait

Implement From<CorsSettings> for Cors trait to configure CORS middleware
with production-ready security validation.

- Move CorsSettings to backend/src/settings/cors.rs module
- Validate wildcard + credentials constraint (browser security policy)
- Configure allowed methods, headers, credentials, and max_age
- Add structured logging for CORS configuration
- Move tests from settings/mod.rs and startup.rs to cors module

Ref: T014

backend/.tarpaulin.local.toml

@@ -4,4 +4,4 @@ skip-clean = true
 target-dir = "coverage"
 output-dir = "coverage"
 fail-under = 60
-exclude-files = ["target/*", "private/*"]
+exclude-files = ["target/*", "private/*", "tests/*"]