fix(app): 前台图片模型 - 分辨率档位与各供应线路取并集，修掉只显示单档

capabilitySpecWithRoutePresets 原先只在「前台自身没有任何尺寸预设」时才与线路合并。但前台快照的预设往往来自单条线路（后续新增的供应线路还没同步进来），条件不成立后，其它线路的 2K/4K 预设永远补不进来，前台只显示 1K。

现在无论自身是否已有预设都与各线路取并集（mergeCapabilityImageSize 已包含前台自身的预设，并集不会丢档）。

线上数据可复现：image-2.0-1k 的前台规格 imageSize.presets 全部是 1k，而它四条线路分别是 none/none/1k/2k+4k。补了一条回归测试，反向验证确认旧逻辑下该测试精确报出 tier 2k missing。

backend/internal/app/logical_models.go

@@ -280,23 +280,21 @@ func capabilitySpecWithRoutePresets(spec CapabilitySpec, routes []CapabilitySpec
 		}
 		result.Options[name] = OptionConstraint{Values: values}
 	}
-	if result.ImageSize == nil || result.ImageSize.Parameter == "" || len(result.ImageSize.Presets) == 0 {
+	// 前台规格里的预设可能只来自单条线路的快照（后续新增的供应线路还没同步进来），
-		if merged := mergeCapabilityImageSize(append([]CapabilitySpec{spec}, routes...)); merged != nil {
+	// 因此不管自身是否已有预设都要与各线路取并集，否则多档会被压成单档。
-			if result.ImageSize == nil {
+	if merged := mergeCapabilityImageSize(append([]CapabilitySpec{spec}, routes...)); merged != nil {
-				result.ImageSize = merged
+		if result.ImageSize == nil {
-			} else {
+			result.ImageSize = merged
-				restored := *result.ImageSize
+		} else {
-				if restored.Parameter == "" {
+			restored := *result.ImageSize
-					restored.Parameter = merged.Parameter
+			if restored.Parameter == "" {
-				}
+				restored.Parameter = merged.Parameter
-				if !restored.AllowCustom {
-					restored.AllowCustom = merged.AllowCustom
-				}
-				if len(restored.Presets) == 0 {
-					restored.Presets = merged.Presets
-				}
-				result.ImageSize = &restored
 			}
+			if !restored.AllowCustom {
+				restored.AllowCustom = merged.AllowCustom
+			}
+			restored.Presets = merged.Presets
+			result.ImageSize = &restored
 		}
 	}
 	return result

backend/internal/app/logical_models_test.go

@@ -26,6 +26,40 @@ func TestCapabilitySpecWithRoutePresetsRestoresImageSize(t *testing.T) {
 	}
 }
 
+func TestCapabilitySpecWithRoutePresetsUnionsTiersAcrossRoutes(t *testing.T) {
+	// 前台快照只带了单条线路的 1K 预设，其它线路提供 2K/4K：必须取并集，不能压成单档。
+	preset := func(size string, tier string, width int, height int) CapabilityImageSizePreset {
+		return CapabilityImageSizePreset{Size: size, Tier: tier, Ratio: "16:9", Width: width, Height: height}
+	}
+	product := CapabilitySpec{
+		Version:    1,
+		Capability: "image",
+		Options:    map[string]OptionConstraint{"size": {Values: []any{"16:9"}}},
+		ImageSize: &CapabilityImageSize{
+			Parameter: "aspect_ratio",
+			Presets:   []CapabilityImageSizePreset{preset("1824x1024", "1k", 1824, 1024)},
+		},
+	}
+	routes := []CapabilitySpec{
+		{Version: 1, Capability: "image", ImageSize: &CapabilityImageSize{Parameter: "aspect_ratio", Presets: []CapabilityImageSizePreset{preset("2752x1536", "2k", 2752, 1536)}}},
+		{Version: 1, Capability: "image", ImageSize: &CapabilityImageSize{Parameter: "aspect_ratio", Presets: []CapabilityImageSizePreset{preset("3840x2160", "4k", 3840, 2160)}}},
+	}
+
+	got := capabilitySpecWithRoutePresets(product, routes)
+	if got.ImageSize == nil {
+		t.Fatal("imageSize = nil")
+	}
+	tiers := make(map[string]bool, len(got.ImageSize.Presets))
+	for _, item := range got.ImageSize.Presets {
+		tiers[item.Tier] = true
+	}
+	for _, tier := range []string{"1k", "2k", "4k"} {
+		if !tiers[tier] {
+			t.Fatalf("tier %s missing: %#v", tier, got.ImageSize.Presets)
+		}
+	}
+}
+
 func TestMatchCapabilityRoutesTextImagesOnlyToDeclaredProfiles(t *testing.T) {
 	plainText := CapabilitySpec{
 		Version:    1,