feat: 实现设置账号并发数功能（支持 bulk-update 与 fallback 逐个更新）

backend/app/schemas/website.py

@@ -166,7 +166,7 @@ class ImportAccountsRequest(BaseModel):
     account_name_prefix: str = "SmartUp"
     default_platform: str = "openai"
     platform_mode: str = "auto"  # "auto" | "manual"
-    concurrency: int = Field(default=10, ge=1)
+    concurrency: int = Field(default=100, ge=1, le=1000)
     priority: int = Field(default=1, ge=0)
     auto_priority_by_rate: bool = True
 
@@ -268,3 +268,22 @@ class CleanupInvalidAccountsExecuteResponse(BaseModel):
     success: bool
     message: str
     items: list[CleanupInvalidAccountsItem]
+
+
+class SetConcurrencyRequest(BaseModel):
+    concurrency: int = Field(default=100, ge=1, le=1000)
+
+
+class SetConcurrencyItem(BaseModel):
+    account_id: str
+    account_name: Optional[str] = None
+    old_concurrency: Optional[int] = None
+    target_concurrency: int
+    status: str  # "success" | "skipped" | "failed"
+    message: str
+
+
+class SetConcurrencyResponse(BaseModel):
+    success: bool
+    message: str
+    items: list[SetConcurrencyItem]