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/wiki.py

@@ -5,8 +5,7 @@ Endpoints for wiki page and dossier management.
 All operations are scoped to user namespaces for multi-tenancy.
 """
 
-from fastapi import APIRouter, HTTPException, Depends, Query, Security, BackgroundTasks
-from fastapi.security import HTTPAuthorizationCredentials
+from fastapi import APIRouter, HTTPException, Depends, Query, BackgroundTasks
 from typing import Optional
 import logging