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

specs/001-modbus-relay-control/tasks.md

@@ -114,7 +114,7 @@
   - **File**: backend/src/startup.rs (in tests module)
   - **Complexity**: Medium | **Uncertainty**: Low
 
-- [ ] **T014** [Setup] [TDD] Implement build_cors() free function in startup.rs
+- [x] **T014** [Setup] [TDD] Implement build_cors() free function in startup.rs
   - Function signature: `fn build_cors(settings: &CorsSettings) -> Cors`
   - Validate: if `allow_credentials=true` AND `allowed_origins` contains "*", panic with clear error message
   - Iterate over `allowed_origins` and call `cors.allow_origin()` for each