feat(ai): 文档管理 handler — CRUD + Multipart 上传

- list_documents: 分页列表（按知识库过滤）
- get_document: 文档详情
- create_manual_document: 手动输入文档
- upload_document: Multipart 文件上传（20MB 限制 + 自动解析）
- delete_document: 软删除（级联减计数）
- 5 条路由注册到 /ai/knowledge-bases/{kb_id}/documents + /ai/documents/*

Phase 2 Task 10

crates/erp-ai/src/module.rs

@@ -609,6 +609,27 @@ impl AiModule {
                 "/ai/knowledge-bases/{id}",
                 axum::routing::delete(crate::handler::knowledge_v2_handler::delete_knowledge_base),
             )
+            // 文档管理路由
+            .route(
+                "/ai/knowledge-bases/{kb_id}/documents",
+                axum::routing::get(crate::handler::document_handler::list_documents),
+            )
+            .route(
+                "/ai/documents/manual",
+                axum::routing::post(crate::handler::document_handler::create_manual_document),
+            )
+            .route(
+                "/ai/documents/upload",
+                axum::routing::post(crate::handler::document_handler::upload_document),
+            )
+            .route(
+                "/ai/documents/{id}",
+                axum::routing::get(crate::handler::document_handler::get_document),
+            )
+            .route(
+                "/ai/knowledge-bases/{kb_id}/documents/{id}",
+                axum::routing::delete(crate::handler::document_handler::delete_document),
+            )
             .route(
                 "/ai/dialysis/risk-assessment",
                 axum::routing::post(crate::handler::assess_dialysis_risk),