feat(core): bind request services and implement registered tasks

Wire plugin services through Bind/InjectFrom and AppContext so HTTP and
workers resolve dependencies after Apply. Register TaskHandler objects
with persisted results, and implement send_email_code, mail:send,
cleanup_inactive_users, and dispatch_bot_msg.

backend/docs/swagger.yaml

@@ -1363,6 +1363,13 @@ definitions:
     - password
     - username
     type: object
+  user.sendEmailCodeRequest:
+    properties:
+      email:
+        type: string
+    required:
+    - email
+    type: object
   user.updateProfileRequest:
     properties:
       avatar_url:
@@ -4937,6 +4944,13 @@ paths:
       consumes:
       - application/json
       description: 向指定邮箱发送验证码（用于注册场景）
+      parameters:
+      - description: 目标邮箱
+        in: body
+        name: request
+        required: true
+        schema:
+          $ref: '#/definitions/user.sendEmailCodeRequest'
       produces:
       - application/json
       responses:
@@ -4948,6 +4962,10 @@ paths:
           description: 参数错误
           schema:
             $ref: '#/definitions/response.Any'
+        "500":
+          description: 发送失败
+          schema:
+            $ref: '#/definitions/response.Any'
       summary: 发送邮箱验证码
       tags:
       - user