refactor: remove dead code and unused imports

- Remove unused get_default_user() from dependencies.py
- Remove unused imports from routers:
  - wiki.py: HTTPAuthorizationCredentials, Security
  - graph.py: Neo4jClient, WikiJSClient
  - hybrid_rag.py: VectorService, GraphService (duplicates)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/core/dependencies.py

@@ -395,18 +395,6 @@ def get_rag_search_service() -> "RAGSearchService":
     )
 
 
-# Utility: Get default user from settings or multi_tenancy
-def get_default_user() -> str:
-    """
-    Get default user for operations.
-
-    Returns:
-        Default user identifier
-    """
-    from src.core.multi_tenancy import DEFAULT_USER
-    return DEFAULT_USER
-
-
 # Authentication
 from fastapi import Security, HTTPException
 from fastapi.security import HTTPBearer