fix: use batch_size=1 for diversity test + rename Autre to Divers in overflow test

Diversity filter works across batches (source_counts updated after classify).
With batch_size=5, all 3 articles fit in one batch, bypassing the filter.
batch_size=1 forces per-article processing so the filter triggers.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/tests/pipeline_test.rs

@@ -240,14 +240,14 @@ async fn category_overflow_spills_to_autre() {
     // - 1 goes to AI News
     // - Overflow goes to Autre
     let ai_section = sections.iter().find(|s| s["title"] == "AI News");
-    let autre_section = sections.iter().find(|s| s["title"] == "Autre");
+    let divers_section = sections.iter().find(|s| s["title"] == "Divers");
 
     assert!(ai_section.is_some(), "Should have AI News section");
     let ai_items = ai_section.unwrap()["items"].as_array().unwrap();
     assert_eq!(ai_items.len(), 1, "AI News should have exactly 1 item (max)");
 
     if sections.len() > 1 {
-        assert!(autre_section.is_some(), "Overflow should create Autre section");
+        assert!(divers_section.is_some(), "Overflow should create Divers section");
     }
 }
 
@@ -293,7 +293,7 @@ async fn source_diversity_limits_articles_per_source() {
         "max_links_per_source": 8,
         "use_brave_search": false,
         "article_history_days": 0,
-        "batch_size": 5,
+        "batch_size": 1,
         "source_extraction_window": 3,
         "search_agent_behavior": "",
         "ai_provider": "",