Add image listing and asset management

Show images in asset directories with preview, rename, replacement, and trash recovery. Update local Markdown references when images move and restore original files if related metadata cannot be committed.

Refresh image previews after replacement and compare actual contents when deduplicating subsequent uploads.

internal/api/openapi.json

@@ -398,7 +398,7 @@
     },
     "/tree": {
       "get": {
-        "summary": "全部笔记与文件夹（不含 .assets 图片目录与隐藏文件）",
+        "summary": "全部笔记、图片与文件夹（含 .assets，不含隐藏条目或符号链接）",
         "tags": [
           "笔记"
         ],
@@ -583,7 +583,7 @@
         }
       },
       "delete": {
-        "summary": "把笔记或文件夹移入回收站（连同 .assets）",
+        "summary": "把笔记、图片或文件夹移入回收站（笔记连同 .assets）",
         "tags": [
           "笔记"
         ],
@@ -613,11 +613,11 @@
     },
     "/move": {
       "post": {
-        "summary": "移动或重命名笔记/文件夹",
+        "summary": "移动或重命名笔记、图片或文件夹",
         "tags": [
           "笔记"
         ],
-        "description": "笔记的 .assets 目录随之移动，改名时同步改写正文中指向自身图片目录的链接；分享链接随之更新。不改写其他笔记指向它的链接。",
+        "description": "笔记的 .assets 目录随之移动，改名时改写本篇图片目录链接；图片移动或改名保持扩展名，并更新仓库笔记中的本地引用。分享链接随主体更新。不改写其他笔记指向笔记或文件夹的链接。",
         "requestBody": {
           "required": true,
           "content": {
@@ -2273,6 +2273,59 @@
           }
         }
       }
+    },
+    "/assets/{path}": {
+      "put": {
+        "summary": "原位替换已有图片，保留路径和引用",
+        "tags": [
+          "图片"
+        ],
+        "parameters": [
+          {
+            "name": "path",
+            "in": "path",
+            "required": true,
+            "description": "仓库中的图片路径",
+            "schema": {
+              "type": "string"
+            }
+          }
+        ],
+        "description": "multipart file 字段的扩展名须与目标图片一致；不创建新文件，最大 50 MB。旧图片不会进入回收站。",
+        "requestBody": {
+          "required": true,
+          "content": {
+            "multipart/form-data": {
+              "schema": {
+                "type": "object",
+                "properties": {
+                  "file": {
+                    "type": "string",
+                    "format": "binary"
+                  }
+                },
+                "required": [
+                  "file"
+                ]
+              }
+            }
+          }
+        },
+        "responses": {
+          "200": {
+            "description": "替换成功"
+          },
+          "400": {
+            "description": "非法路径、空文件或扩展名不一致"
+          },
+          "404": {
+            "description": "目标图片不存在"
+          },
+          "413": {
+            "description": "超过 50 MB"
+          }
+        }
+      }
     }
   },
   "components": {