feat(ci): add format check, PR validation, and Docker buildx

- Add cargo fmt --check to enforce code formatting
- Add pull_request trigger for PR validation
- Split workflow into parallel jobs (format, clippy, build, docker)
- Integrate Docker Buildx with DinD service
- Add BuildKit caching for faster builds
- Add local test script (scripts/test-ci-locally.sh)
- Add comprehensive documentation

All local CI checks pass ✅

backend/src/services/interaction_service.rs

@@ -135,7 +135,7 @@ mod tests {
         println!("Found {} interactions", result.interactions.len());
         println!("Note: This test verifies the API works correctly");
         println!("       The actual interaction database may need more entries");
-        
+
         // For now, just verify the API returns a valid response
         assert!(!result.disclaimer.is_empty());
     }