feat(tenant): enforce English output for en-US template generation

When the article locale is en-US, Chinese prompt templates could leak
Chinese wording into generated titles, outlines, and article bodies.
Add high-priority language-consistency guards so the model treats the
Chinese template text as internal instructions and emits English only.

- Append per-artifact locale guards (analyze/title/outline) in template
  assist prompts and an article-body guard in the generation prompt,
  only when locale is en-US
- Reinforce locale consistency in the title/outline runtime prompts and
  drop retained images from the rewrite prompt
- Cover the new guards with unit tests

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

server/internal/tenant/app/template_assist_test.go

@@ -64,6 +64,64 @@ func TestBuildOutlinePromptForConfiguredTemplateForcesOutlineObject(t *testing.T
 	}
 }
 
+func TestBuildAnalyzePromptEnglishLocaleAddsHardLanguageGuard(t *testing.T) {
+	template := "你正在做品牌分析。\n语言: {{locale}}\n品牌名: {{brand_name}}"
+
+	prompt := buildAnalyzePrompt("top_x_article", "Top X 推荐文章", AnalyzeTaskRequest{
+		Locale:    "en-US",
+		BrandName: "Rankly",
+	}, &template)
+
+	for _, expected := range []string{
+		"High-priority language consistency requirement:",
+		"Return English JSON field values for brand_summary, keywords, competitor descriptions",
+		"Competitor, brand, company, and product names may remain in their official language",
+	} {
+		if !strings.Contains(prompt, expected) {
+			t.Fatalf("buildAnalyzePrompt() = %q, want English language guard %q", prompt, expected)
+		}
+	}
+}
+
+func TestBuildTitlePromptEnglishLocaleAddsHardLanguageGuard(t *testing.T) {
+	template := "你正在为文章生成标题。\n语言: {{locale}}\n优化关键词: {{primary_keyword}}"
+
+	prompt := buildTitlePrompt("top_x_article", "Top X 推荐文章", TitleTaskRequest{
+		Locale:        "en-US",
+		BrandQuestion: "whole-home customization",
+	}, &template)
+
+	for _, expected := range []string{
+		"High-priority language consistency requirement:",
+		"Return English JSON strings only.",
+		"The titles must be natural, professional English titles",
+	} {
+		if !strings.Contains(prompt, expected) {
+			t.Fatalf("buildTitlePrompt() = %q, want English language guard %q", prompt, expected)
+		}
+	}
+}
+
+func TestBuildOutlinePromptEnglishLocaleAddsHardLanguageGuard(t *testing.T) {
+	template := "你正在为文章生成大纲。\n语言: {{locale}}\n标题: {{title}}\n已选段落: {{outline_sections}}"
+
+	prompt := buildOutlinePrompt("top_x_article", "Top X 推荐文章", OutlineTaskRequest{
+		Locale:          "en-US",
+		Title:           "How to Choose Whole-home Customization",
+		OutlineSections: []string{"Introduction", "Key Points"},
+	}, &template)
+
+	for _, expected := range []string{
+		"High-priority language consistency requirement:",
+		"Every outline and child outline value must be natural, professional English.",
+		"Do not output Chinese section names",
+	} {
+		if !strings.Contains(prompt, expected) {
+			t.Fatalf("buildOutlinePrompt() = %q, want English language guard %q", prompt, expected)
+		}
+	}
+}
+
 func TestNormalizeAnalyzeResultKeepsTenQueryKeywords(t *testing.T) {
 	input := AnalyzeTaskResult{
 		BrandSummary: "  全屋定制服务摘要  ",