fix: return 204 No Content from preferred sources endpoint

The API client expects empty responses to use 204, not 200.
Returning 200 with no body caused JSON parse error in the frontend.

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

backend/src/handlers/sources.rs

@@ -324,5 +324,5 @@ pub async fn update_preferred(
     }
 
     db::sources::update_preferred(&state.pool, auth_user.id, &body.source_ids, body.theme_id).await?;
-    Ok(StatusCode::OK)
+    Ok(StatusCode::NO_CONTENT)
 }

backend/tests/api_sources_preferred_test.rs

@@ -62,7 +62,7 @@ async fn update_preferred_sets_sources() {
     let (pref_status, _) = app
         .put_with_session("/api/v1/sources/preferred", &pref_body, &session)
         .await;
-    assert_eq!(pref_status, StatusCode::OK);
+    assert_eq!(pref_status, StatusCode::NO_CONTENT);
 
     // GET /sources → verify preferred flags
     let (list_status, list_body) = app
@@ -132,7 +132,7 @@ async fn update_preferred_clears_all_when_empty() {
     let (pref_status, _) = app
         .put_with_session("/api/v1/sources/preferred", &pref_body, &session)
         .await;
-    assert_eq!(pref_status, StatusCode::OK);
+    assert_eq!(pref_status, StatusCode::NO_CONTENT);
 
     // Clear all preferred
     let clear_body = serde_json::json!({

backend/tests/pipeline_test.rs

@@ -638,7 +638,7 @@ async fn preferred_sources_processed_first() {
     let (pref_status, _) = app
         .put_with_session("/api/v1/sources/preferred", &pref_body, &session)
         .await;
-    assert_eq!(pref_status.as_u16(), 200, "Setting preferred sources should succeed");
+    assert_eq!(pref_status.as_u16(), 204, "Setting preferred sources should succeed");
 
     // Run the pipeline
     let mock_provider = MockLlmProvider::new()

e2e/tests/sources.spec.ts

@@ -132,7 +132,7 @@ test.describe('Sources management', () => {
       });
       return { status: resp.status };
     }, { ids: [sourceId2], tid: themeId });
-    expect(prefResp.status).toBe(200);
+    expect(prefResp.status).toBe(204);
 
     // Step 9: Verify source is now preferred
     const listResp3 = await page.evaluate(async (tid: string) => {