refactor(layout): consolidate backend codebase into backend/ package and clean root directory

- Moved cmd/, core/, plugins/, pkg/, downstream/, and main.go into backend/ directory
- Batch updated all Go source files to import github.com/Rain-kl/Wavelet/backend/...
- Updated Makefile, scripts/swagger.sh, architecture guards, and platform skills
- Passed all quality gates (100% tests, 0 lint issues, clean build)

docs/swagger.yaml

@@ -628,6 +628,34 @@ definitions:
       website:
         type: string
     type: object
+  cap.ChallengeResponse:
+    properties:
+      challenge:
+        properties:
+          c:
+            type: integer
+          d:
+            type: integer
+          s:
+            type: integer
+        type: object
+      expires:
+        description: ms timestamp
+        type: integer
+      token:
+        type: string
+    type: object
+  cap.RedeemResponse:
+    properties:
+      error:
+        type: string
+      expires:
+        type: integer
+      success:
+        type: boolean
+      token:
+        type: string
+    type: object
   cap.challengeRequest:
     properties:
       scope:
@@ -662,32 +690,49 @@ definitions:
       ttl_minutes:
         type: integer
     type: object
-  github_com_Rain-kl_Wavelet_plugins_domain_cap.ChallengeResponse:
+  driver_asynq_worker.TaskMeta:
     properties:
-      challenge:
-        properties:
-          c:
-            type: integer
-          d:
-            type: integer
-          s:
-            type: integer
-        type: object
-      expires:
-        description: ms timestamp
+      asynq_task:
+        type: string
+      description:
+        type: string
+      max_retry:
         type: integer
-      token:
+      name:
+        type: string
+      params:
+        items:
+          $ref: '#/definitions/driver_asynq_worker.TaskParam'
+        type: array
+      queue:
+        type: string
+      retryable:
+        description: 是否支持手动重试
+        type: boolean
+      supports_time:
+        type: boolean
+      type:
         type: string
     type: object
-  github_com_Rain-kl_Wavelet_plugins_domain_cap.RedeemResponse:
+  driver_asynq_worker.TaskParam:
     properties:
-      error:
+      description:
+        description: 描述
         type: string
-      expires:
-        type: integer
-      success:
+      label:
+        description: 显示名称
+        type: string
+      name:
+        description: 参数键名
+        type: string
+      placeholder:
+        description: 占位符
+        type: string
+      required:
+        description: 是否必填
         type: boolean
-      token:
+      type:
+        description: 类型：string, text, number, boolean
         type: string
     type: object
   handler.batchDownloadRequest:
@@ -973,51 +1018,6 @@ definitions:
         example: ""
         type: string
     type: object
-  task.TaskMeta:
-    properties:
-      asynq_task:
-        type: string
-      description:
-        type: string
-      max_retry:
-        type: integer
-      name:
-        type: string
-      params:
-        items:
-          $ref: '#/definitions/task.TaskParam'
-        type: array
-      queue:
-        type: string
-      retryable:
-        description: 是否支持手动重试
-        type: boolean
-      supports_time:
-        type: boolean
-      type:
-        type: string
-    type: object
-  task.TaskParam:
-    properties:
-      description:
-        description: 描述
-        type: string
-      label:
-        description: 显示名称
-        type: string
-      name:
-        description: 参数键名
-        type: string
-      placeholder:
-        description: 占位符
-        type: string
-      required:
-        description: 是否必填
-        type: boolean
-      type:
-        description: 类型：string, text, number, boolean
-        type: string
-    type: object
 info:
   contact:
     name: Wavelet
@@ -1050,7 +1050,7 @@ paths:
             - $ref: '#/definitions/response.Any'
             - properties:
                 data:
-                  $ref: '#/definitions/github_com_Rain-kl_Wavelet_plugins_domain_cap.ChallengeResponse'
+                  $ref: '#/definitions/cap.ChallengeResponse'
               type: object
         "500":
           description: 内部服务错误
@@ -1081,7 +1081,7 @@ paths:
             - $ref: '#/definitions/response.Any'
             - properties:
                 data:
-                  $ref: '#/definitions/github_com_Rain-kl_Wavelet_plugins_domain_cap.RedeemResponse'
+                  $ref: '#/definitions/cap.RedeemResponse'
               type: object
         "400":
           description: 参数错误或核销失败
@@ -2305,7 +2305,7 @@ paths:
             - properties:
                 data:
                   items:
-                    $ref: '#/definitions/task.TaskMeta'
+                    $ref: '#/definitions/driver_asynq_worker.TaskMeta'
                   type: array
               type: object
         "401":