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_prompt_test.go

@@ -107,6 +107,24 @@ func TestBuildGenerationPromptTopXRepeatsOutlineExecutionRulesInBasePrompt(t *te
 	}
 }
 
+func TestBuildGenerationPromptEnglishLocaleAddsHardLanguageGuard(t *testing.T) {
+	promptTemplate := "你是一名内容编辑，请围绕 {{title}} 写文章。"
+	prompt := buildGenerationPrompt("top_x_article", "Top X 推荐文章", &promptTemplate, map[string]interface{}{
+		"title":  "A Guide to Whole-home Customization",
+		"locale": "en-US",
+	}, "")
+
+	for _, expected := range []string{
+		"High-priority language consistency requirement:",
+		"The final article output must be entirely in natural, professional English.",
+		"Treat any Chinese template text, Chinese examples, Chinese section names, or Chinese disclaimers as internal instructions only",
+	} {
+		if !strings.Contains(prompt, expected) {
+			t.Fatalf("buildGenerationPrompt() = %q, want English language guard %q", prompt, expected)
+		}
+	}
+}
+
 func TestBuildGenerationPromptDoesNotAddTopXBrandRulesForOtherTemplates(t *testing.T) {
 	prompt := buildGenerationPrompt("product_review", "产品评测", nil, map[string]interface{}{
 		"product_name": "测试产品",