feat: RAG inline citations, source highlighting, and admin panel

- Increase RAG retrieval from 5 to 50 docs with relevance threshold (0.15)
- LLM inline citations with [来源N] format and reference list
- Clickable citation links scroll to source cards with highlight animation
- Source previews with full chunk text and answer-matched highlighting
- Message-scoped source IDs to fix cross-round citation targeting
- Admin panel pages (knowledge, users, forum, course, settings)
- Add rehype-raw dependency for HTML-in-markdown rendering

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/main.py

@@ -185,6 +185,10 @@ from src.api import course_content, forum
 app.include_router(course_content.router)
 app.include_router(forum.router)
 
+# 导入并注册后台管理API
+from src.api import admin
+app.include_router(admin.router)
+
 # 静态文件服务
 if os.path.exists(settings.upload_dir):
     app.mount("/uploads", StaticFiles(directory=settings.upload_dir), name="uploads")