feat(upstreams): add batch test-all / check-now-all endpoints

- POST /api/upstreams/test-all: batch connection test for all enabled
  upstreams (no snapshot, no webhook); updates last_status, balance
- POST /api/upstreams/check-now-all: full batch sync (snapshot, diff,
  webhook, key sync, priority sync); mirrors single check-now behavior
- Both routes are registered before /{uid} to avoid path capture
- Skips disabled upstreams (status=skipped); single failure does not
  abort subsequent upstreams (serial execution)
- Returns UpstreamBatchActionResponse with per-item detail and summary

Refactor: extract _test_upstream_core(db, u) and _check_now_core(db, u)
- All four routes (single + batch × 2) now share the same core helpers
- Eliminates duplicate logic and future divergence risk

Frontend:
- Add UpstreamBatchActionResponse / Item / Summary TS types
- Add upstreamsApi.testAll() and upstreamsApi.checkNowAll()
- Add '一键测试' and '一键同步' buttons in Upstreams.vue toolbar
  (order: 一键测试 → 一键同步 → 刷新 → 新增上游)
- Buttons disabled when list is empty or another batch op is running
- On completion: refresh list + ElMessageBox with per-item failure detail

backend/app/schemas/upstream.py

@@ -128,3 +128,25 @@ class GenerateKeysByGroupsResponse(BaseModel):
     success: bool
     message: str
     items: list[GeneratedUpstreamKeyResponse]
+
+
+class UpstreamBatchActionItem(BaseModel):
+    upstream_id: int
+    upstream_name: str
+    status: str  # success | failed | skipped
+    message: str
+    detail: Optional[str] = None
+
+
+class UpstreamBatchActionSummary(BaseModel):
+    total: int
+    success: int
+    failed: int
+    skipped: int
+
+
+class UpstreamBatchActionResponse(BaseModel):
+    success: bool
+    message: str
+    summary: UpstreamBatchActionSummary
+    items: list[UpstreamBatchActionItem]