feat(imitation): accept pasted source content as an alternative to URL

- Make source_url optional and add source_content to the request; the
  two are normalized into a source kind ("url" | "content"), URL wins
  when both are present, and missing both returns source_required
- Skip web fetching in the generation worker when pasted content is
  provided; truncate it to the same 24,000-rune cap
- Label untitled pasted-content jobs with a locale-aware display title
  and mark the source kind in wizard state and prompt metadata
- Cover source normalization, truncation, and display title with tests

server/internal/tenant/app/article_imitation_service_test.go

@@ -0,0 +1,87 @@
+package app
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestNormalizeImitationSourcePrefersURL(t *testing.T) {
+	t.Parallel()
+
+	sourceURL, sourceContent, sourceKind, err := normalizeImitationSource(GenerateImitationRequest{
+		SourceURL:     " https://example.com/article ",
+		SourceContent: "pasted content should be ignored",
+	})
+	if err != nil {
+		t.Fatalf("normalizeImitationSource() error = %v", err)
+	}
+	if sourceURL != "https://example.com/article" {
+		t.Fatalf("sourceURL = %q, want normalized URL", sourceURL)
+	}
+	if sourceContent != "" {
+		t.Fatalf("sourceContent = %q, want empty when URL is present", sourceContent)
+	}
+	if sourceKind != articleImitationSourceURL {
+		t.Fatalf("sourceKind = %q, want %q", sourceKind, articleImitationSourceURL)
+	}
+}
+
+func TestNormalizeImitationSourceAcceptsPastedContent(t *testing.T) {
+	t.Parallel()
+
+	sourceURL, sourceContent, sourceKind, err := normalizeImitationSource(GenerateImitationRequest{
+		SourceContent: "  article body  ",
+	})
+	if err != nil {
+		t.Fatalf("normalizeImitationSource() error = %v", err)
+	}
+	if sourceURL != "" {
+		t.Fatalf("sourceURL = %q, want empty", sourceURL)
+	}
+	if sourceContent != "article body" {
+		t.Fatalf("sourceContent = %q, want trimmed content", sourceContent)
+	}
+	if sourceKind != articleImitationSourceContent {
+		t.Fatalf("sourceKind = %q, want %q", sourceKind, articleImitationSourceContent)
+	}
+}
+
+func TestNormalizeImitationSourceRejectsMissingSource(t *testing.T) {
+	t.Parallel()
+
+	_, _, _, err := normalizeImitationSource(GenerateImitationRequest{})
+	if err == nil {
+		t.Fatal("normalizeImitationSource() error = nil, want source_required")
+	}
+	if err.Error() != "source_required" {
+		t.Fatalf("normalizeImitationSource() error = %q, want source_required", err.Error())
+	}
+}
+
+func TestNormalizeImitationSourceTruncatesPastedContent(t *testing.T) {
+	t.Parallel()
+
+	_, sourceContent, _, err := normalizeImitationSource(GenerateImitationRequest{
+		SourceContent: strings.Repeat("文", maxImitationSourceContentRunes+100),
+	})
+	if err != nil {
+		t.Fatalf("normalizeImitationSource() error = %v", err)
+	}
+	if got := len([]rune(sourceContent)); got != maxImitationSourceContentRunes {
+		t.Fatalf("source content rune count = %d, want %d", got, maxImitationSourceContentRunes)
+	}
+}
+
+func TestImitationSourceDisplayTitleLabelsUntitledPastedContent(t *testing.T) {
+	t.Parallel()
+
+	if got := imitationSourceDisplayTitle(articleImitationSourceContent, "", "zh-CN"); got != "粘贴正文" {
+		t.Fatalf("Chinese display title = %q, want 粘贴正文", got)
+	}
+	if got := imitationSourceDisplayTitle(articleImitationSourceContent, "", "en-US"); got != "Pasted content" {
+		t.Fatalf("English display title = %q, want Pasted content", got)
+	}
+	if got := imitationSourceDisplayTitle(articleImitationSourceContent, "Original title", "en-US"); got != "Original title" {
+		t.Fatalf("explicit display title = %q, want Original title", got)
+	}
+}