Replace the 1095-line monolithic SearchService with a thin facade that
delegates to SearchOrchestrator. Split the implementation into three
focused modules:
- search_orchestrator.py: request parsing, provider coordination, and
response construction
- search_providers.py: per-source execution, progress callbacks, and
async task creation
- search_cache.py: timestamping and stable cache stamping for results
Add unit tests for the new components in
tests/test_search_service_components.py. Preserve the existing HTTP
route behavior by keeping SearchService.search_sources as the stable
route-facing API.
