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/routers/graph.py

@@ -15,8 +15,6 @@ from src.models.graph import (
     MindMapResponse
 )
 from src.services.graph_service import GraphService
-from src.clients.neo4j_client import Neo4jClient
-from src.clients.wikijs_client import WikiJSClient
 from src.core.dependencies import Neo4jDep, WikiJSDep, verify_api_key
 from src.core.multi_tenancy import DEFAULT_USER