feat: Add workspace isolation support for pipeline status

Problem:
In multi-tenant scenarios, different workspaces share a single global
pipeline_status namespace, causing pipelines from different tenants to
block each other, severely impacting concurrent processing performance.

Solution:
- Extended get_namespace_data() to recognize workspace-specific pipeline
  namespaces with pattern "{workspace}:pipeline" (following GraphDB pattern)
- Added workspace parameter to initialize_pipeline_status() for per-tenant
  isolated pipeline namespaces
- Updated all 7 call sites to use workspace-aware locks:
  * lightrag.py: process_document_queue(), aremove_document()
  * document_routes.py: background_delete_documents(), clear_documents(),
    cancel_pipeline(), get_pipeline_status(), delete_documents()

Impact:
- Different workspaces can process documents concurrently without blocking
- Backward compatible: empty workspace defaults to "pipeline_status"
- Maintains fail-fast: uninitialized pipeline raises clear error
- Expected N× performance improvement for N concurrent tenants

Bug fixes:
- Fixed AttributeError by using self.workspace instead of self.global_config
- Fixed pipeline status endpoint to show workspace-specific status
- Fixed delete endpoint to check workspace-specific busy flag

Code changes: 4 files, 141 insertions(+), 28 deletions(-)

Testing: All syntax checks passed, comprehensive workspace isolation tests completed

.gitignore

@@ -72,3 +72,5 @@ download_models_hf.py
 
 # Cline files
 memory-bank
+.claude/CLAUDE.md
+.claude/