feat(openai): port /responses/compact account support flow (PR #1555)

将 vansour/sub2api#1555 的 OpenAI compact 能力建模手工移植到当前 main：账号
级 compact 状态/auto-force_on-force_off 模式、compact-only 模型映射、调度器
tier 分层（已支持 > 未知 > 已知不支持）、管理后台 compact 主动探测，以及对应
i18n/状态徽章。普通 /responses 流量行为不变，无数据库迁移。

backend/internal/service/openai_compact_probe_test.go

@@ -0,0 +1,122 @@
+package service
+
+import (
+	"errors"
+	"net/http"
+	"testing"
+	"time"
+)
+
+func TestNormalizeAccountTestMode(t *testing.T) {
+	tests := []struct {
+		input string
+		want  string
+	}{
+		{input: "", want: AccountTestModeDefault},
+		{input: "default", want: AccountTestModeDefault},
+		{input: " compact ", want: AccountTestModeCompact},
+		{input: "COMPACT", want: AccountTestModeCompact},
+		{input: "unknown", want: AccountTestModeDefault},
+	}
+
+	for _, tt := range tests {
+		if got := normalizeAccountTestMode(tt.input); got != tt.want {
+			t.Fatalf("normalizeAccountTestMode(%q) = %q, want %q", tt.input, got, tt.want)
+		}
+	}
+}
+
+func TestBuildOpenAICompactProbeExtraUpdates_SuccessMarksSupported(t *testing.T) {
+	now := time.Date(2026, 4, 10, 10, 0, 0, 0, time.UTC)
+	updates := buildOpenAICompactProbeExtraUpdates(&http.Response{StatusCode: http.StatusOK}, []byte(`{"id":"cmp_1"}`), nil, now)
+
+	if got := updates["openai_compact_supported"]; got != true {
+		t.Fatalf("openai_compact_supported = %v, want true", got)
+	}
+	if got := updates["openai_compact_last_status"]; got != http.StatusOK {
+		t.Fatalf("openai_compact_last_status = %v, want %d", got, http.StatusOK)
+	}
+	if got := updates["openai_compact_last_error"]; got != "" {
+		t.Fatalf("openai_compact_last_error = %v, want empty string", got)
+	}
+	if got := updates["openai_compact_checked_at"]; got != now.Format(time.RFC3339) {
+		t.Fatalf("openai_compact_checked_at = %v, want %s", got, now.Format(time.RFC3339))
+	}
+}
+
+func TestBuildOpenAICompactProbeExtraUpdates_404MarksUnsupported(t *testing.T) {
+	now := time.Date(2026, 4, 10, 10, 0, 0, 0, time.UTC)
+	body := []byte(`404 page not found`)
+	updates := buildOpenAICompactProbeExtraUpdates(&http.Response{StatusCode: http.StatusNotFound}, body, nil, now)
+
+	if got := updates["openai_compact_supported"]; got != false {
+		t.Fatalf("openai_compact_supported = %v, want false", got)
+	}
+	if got := updates["openai_compact_last_status"]; got != http.StatusNotFound {
+		t.Fatalf("openai_compact_last_status = %v, want %d", got, http.StatusNotFound)
+	}
+}
+
+func TestBuildOpenAICompactProbeExtraUpdates_502DoesNotMarkUnsupported(t *testing.T) {
+	now := time.Date(2026, 4, 10, 10, 0, 0, 0, time.UTC)
+	updates := buildOpenAICompactProbeExtraUpdates(&http.Response{StatusCode: http.StatusBadGateway}, []byte(`Upstream request failed`), nil, now)
+
+	if _, exists := updates["openai_compact_supported"]; exists {
+		t.Fatalf("did not expect openai_compact_supported for 502 response")
+	}
+	if got := updates["openai_compact_last_status"]; got != http.StatusBadGateway {
+		t.Fatalf("openai_compact_last_status = %v, want %d", got, http.StatusBadGateway)
+	}
+}
+
+func TestBuildOpenAICompactProbeExtraUpdates_RequestErrorDoesNotMarkUnsupported(t *testing.T) {
+	now := time.Date(2026, 4, 10, 10, 0, 0, 0, time.UTC)
+	updates := buildOpenAICompactProbeExtraUpdates(nil, nil, errors.New("dial tcp timeout"), now)
+
+	if _, exists := updates["openai_compact_supported"]; exists {
+		t.Fatalf("did not expect openai_compact_supported for request error")
+	}
+	if got, exists := updates["openai_compact_last_status"]; !exists || got != nil {
+		t.Fatalf("openai_compact_last_status = %v, want nil key", got)
+	}
+	if got := updates["openai_compact_last_error"]; got == "" {
+		t.Fatalf("expected openai_compact_last_error to be populated")
+	}
+}
+
+func TestBuildOpenAICompactProbeExtraUpdates_NoResponseClearsLastStatus(t *testing.T) {
+	now := time.Date(2026, 4, 10, 10, 0, 0, 0, time.UTC)
+	updates := buildOpenAICompactProbeExtraUpdates(nil, nil, nil, now)
+
+	if got, exists := updates["openai_compact_last_status"]; !exists || got != nil {
+		t.Fatalf("openai_compact_last_status = %v, want nil key", got)
+	}
+	if got := updates["openai_compact_last_error"]; got != "compact probe failed" {
+		t.Fatalf("openai_compact_last_error = %v, want compact probe failed", got)
+	}
+}
+
+func TestBuildOpenAICompactProbeExtraUpdates_UnknownModelDoesNotMarkUnsupported(t *testing.T) {
+	now := time.Date(2026, 4, 10, 10, 0, 0, 0, time.UTC)
+	body := []byte(`{"error":{"message":"unknown model gpt-5.4-openai-compact"}}`)
+	updates := buildOpenAICompactProbeExtraUpdates(&http.Response{StatusCode: http.StatusBadRequest}, body, nil, now)
+
+	if _, exists := updates["openai_compact_supported"]; exists {
+		t.Fatalf("did not expect openai_compact_supported for unknown-model diagnostics")
+	}
+	if got := updates["openai_compact_last_status"]; got != http.StatusBadRequest {
+		t.Fatalf("openai_compact_last_status = %v, want %d", got, http.StatusBadRequest)
+	}
+}
+
+func TestBuildOpenAICompactProbeExtraUpdates_EmptyFailureBodyFallsBackToHTTPStatus(t *testing.T) {
+	now := time.Date(2026, 4, 10, 10, 0, 0, 0, time.UTC)
+	updates := buildOpenAICompactProbeExtraUpdates(&http.Response{StatusCode: http.StatusServiceUnavailable}, nil, nil, now)
+
+	if got := updates["openai_compact_last_status"]; got != http.StatusServiceUnavailable {
+		t.Fatalf("openai_compact_last_status = %v, want %d", got, http.StatusServiceUnavailable)
+	}
+	if got := updates["openai_compact_last_error"]; got != "HTTP 503" {
+		t.Fatalf("openai_compact_last_error = %v, want HTTP 503", got)
+	}
+}