perf(server): Medium-term stability and long-term OCP relations registry optimizations

[arvarik]

This pull request implements the approved backend optimizations for medium-term stability and long-term maintainability:

1. Event-loop yielding in the deduplication scanning composite scoring loop via setImmediate, preventing single-threaded Express loops from freezing.
2. Sequential bulk background queue processing during standard JSON imports utilizing the existing ParallelQueue (concurrency=1), resolving transactional lock contention.
3. OCP relationship registry centralizing all child record definitions in contactRepository.ts and dynamically mapping update iterations in contactService.ts, making the updates layer robust to new child tables.
4. Regression Unit Testing: Created a dedicated unit test suite for the RELATION_REGISTRY verifying that all 10 core relation tables are correctly configured.

All 182 test cases pass successfully, with zero linter warnings and zero compilation errors.