feat: extract recent domains and pass to search prompt for diversity

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/services/synthesis.rs

@@ -300,8 +300,42 @@ async fn run_generation_inner(
     let current_date = Utc::now()
         .format("%A %d %B %Y")
         .to_string();
+    // Step 5b: Load recently-used domains for source diversity
+    let recent_domains = if settings.source_diversity_window > 0 {
+        let recent = db::syntheses::list_for_user(
+            &state.pool,
+            user_id,
+            settings.source_diversity_window as i64,
+            0,
+        )
+        .await
+        .unwrap_or_default();
+
+        let mut domains: Vec<String> = recent
+            .iter()
+            .filter_map(|s| {
+                serde_json::from_value::<Vec<crate::models::synthesis::NewsSection>>(
+                    s.sections.clone(),
+                )
+                .ok()
+            })
+            .flat_map(|sections| {
+                sections
+                    .into_iter()
+                    .flat_map(|sec| sec.items.into_iter())
+                    .filter_map(|item| extract_domain(&item.url))
+            })
+            .collect();
+
+        domains.sort();
+        domains.dedup();
+        domains
+    } else {
+        Vec::new()
+    };
+
     let (system_prompt, user_prompt) =
-        prompts::build_search_prompt(&settings, &sources, &current_date, &[]);
+        prompts::build_search_prompt(&settings, &sources, &current_date, &recent_domains);
 
     let raw_results = provider
         .generate_search_pass(&model_research, &system_prompt, &user_prompt, &schema)