feat: 产品内知识库（obsidian-wiki 兼容 vault）

后端：新增自包含 deerflow.knowledge 模块（models/repository/schemas/
extractor/sources/markdown_vault/manifest/obsidian_adapter/exporter/
search/service），通过 deps.py 装配为 app.state.knowledge_service，
挂载 /api/knowledge 路由；config.yaml 增加 knowledge 配置段，
AppConfig 注册 KnowledgeConfig，ORM 注册 4 张 knowledge_* 表。

前端：新增知识库页面（KnowledgeBasePage/KnowledgeGraphPage）、
core/knowledge api/hooks/types、聊天侧 knowledge-save-trigger，
侧边栏增加「知识库」入口。

知识库为全局共享，不做 user_id 隔离，写入记录 created_by/updated_by
作为审计。

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

offline-backend-20260512/backend/app/gateway/app.py

@@ -38,6 +38,7 @@ from app.gateway.routers import (
     feedback,
     html_page_favorites,
     intent,
+    knowledge,
     llm_metrics,
     mcp,
     memory,
@@ -1252,6 +1253,9 @@ This gateway provides custom endpoints for models, MCP configuration, skills, an
     app.include_router(public_embed.router)
     app.include_router(html_page_favorites.router)
 
+    # Product knowledge base API is mounted at /api/knowledge
+    app.include_router(knowledge.router)
+
     # System settings API is mounted at /api/system-settings
     app.include_router(system_settings.router)