chore(webapp): downgrade SVE logs in api.v1.projects.background-workers to warn

ServiceValidationError and CreateDeclarativeScheduleError are user-facing
validation failures returned as 4xx; they shouldn't surface in Sentry. Move
type discrimination before the log call and split the SVE/CDSE branches at
warn level. Real 5xx errors continue to log at error.

Refs the ignoreErrors filter added in dac9c83; closes the wrapper-bypass
gap on TRIGGER-CLOUD-2S.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: d-cs

apps/webapp/app/routes/api.v1.projects.$projectRef.background-workers.ts

@@ -58,14 +58,20 @@ export async function action({ request, params }: ActionFunctionArgs) {
       { status: 200 }
     );
   } catch (e) {
-    logger.error("Failed to create background worker", { error: JSON.stringify(e) });
-
+    // Customer-facing validation failures (invalid task config, customer cron
+    // expression, etc.). The handler returns 4xx with the message; system
+    // handles it gracefully, no alert needed.
     if (e instanceof ServiceValidationError) {
+      logger.warn("Failed to create background worker", { error: e.message });
       return json({ error: e.message }, { status: 400 });
-    } else if (e instanceof CreateDeclarativeScheduleError) {
+    }
+    if (e instanceof CreateDeclarativeScheduleError) {
+      logger.warn("Failed to create background worker", { error: e.message });
       return json({ error: e.message }, { status: 400 });
     }
 
+    logger.error("Failed to create background worker", { error: e });
+
     return json({ error: "Failed to create background worker" }, { status: 500 });
   }
 }