feat: 添加媒体资源管理相关API接口及数据结构，包括列表和详情查询

backend/docs/swagger.yaml

@@ -1921,6 +1921,112 @@ paths:
       summary: 拒绝加入申请（租户管理）
       tags:
       - Tenant
+  /t/{tenantCode}/v1/admin/media_assets:
+    get:
+      consumes:
+      - application/json
+      parameters:
+      - description: Tenant Code
+        in: path
+        name: tenantCode
+        required: true
+        type: string
+      - description: Asc specifies comma-separated field names to sort ascending by.
+        in: query
+        name: asc
+        type: string
+      - description: CreatedAtFrom filters assets by created_at >= this time; optional.
+        in: query
+        name: created_at_from
+        type: string
+      - description: CreatedAtTo filters assets by created_at <= this time; optional.
+        in: query
+        name: created_at_to
+        type: string
+      - description: Desc specifies comma-separated field names to sort descending
+          by.
+        in: query
+        name: desc
+        type: string
+      - description: Limit is page size; only values in {10,20,50,100} are accepted
+          (otherwise defaults to 10).
+        in: query
+        name: limit
+        type: integer
+      - description: Page is 1-based page index; values <= 0 are normalized to 1.
+        in: query
+        name: page
+        type: integer
+      - description: Status filters by processing status (uploaded/processing/ready/failed/deleted);
+          optional.
+        enum:
+        - uploaded
+        - processing
+        - ready
+        - failed
+        - deleted
+        in: query
+        name: status
+        type: string
+        x-enum-varnames:
+        - MediaAssetStatusUploaded
+        - MediaAssetStatusProcessing
+        - MediaAssetStatusReady
+        - MediaAssetStatusFailed
+        - MediaAssetStatusDeleted
+      - description: Type filters by media type (video/audio/image); optional.
+        enum:
+        - video
+        - audio
+        - image
+        in: query
+        name: type
+        type: string
+        x-enum-varnames:
+        - MediaAssetTypeVideo
+        - MediaAssetTypeAudio
+        - MediaAssetTypeImage
+      produces:
+      - application/json
+      responses:
+        "200":
+          description: OK
+          schema:
+            allOf:
+            - $ref: '#/definitions/requests.Pager'
+            - properties:
+                items:
+                  $ref: '#/definitions/models.MediaAsset'
+              type: object
+      summary: 媒体资源列表（租户管理）
+      tags:
+      - Tenant
+  /t/{tenantCode}/v1/admin/media_assets/{assetID}:
+    get:
+      consumes:
+      - application/json
+      parameters:
+      - description: Tenant Code
+        in: path
+        name: tenantCode
+        required: true
+        type: string
+      - description: AssetID
+        format: int64
+        in: path
+        name: assetID
+        required: true
+        type: integer
+      produces:
+      - application/json
+      responses:
+        "200":
+          description: OK
+          schema:
+            $ref: '#/definitions/models.MediaAsset'
+      summary: 媒体资源详情（租户管理）
+      tags:
+      - Tenant
   /t/{tenantCode}/v1/admin/media_assets/{assetID}/upload_complete:
     post:
       consumes: