chore(webapp): downgrade SVE/Invalid-JSON logs in api.v3.batches.items to warn

Closes the wrapper-bypass gap on TRIGGER-CLOUD-103 (Stream batch items
error, 1052 events in 48h). Customer-facing 4xx paths (invalid item
shape, invalid JSON in stream body) now log at warn; only the 500
fall-through escalates to Sentry.

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

Author: d-cs

apps/webapp/app/routes/api.v3.batches.$batchId.items.ts

@@ -88,6 +88,22 @@ export async function action({ request, params }: ActionFunctionArgs) {
 
     return json(result, { status: 200 });
   } catch (error) {
+    // Customer-facing validation failures (invalid item shape, invalid JSON
+    // in the streamed body). The handler returns 4xx with the message;
+    // system handles it gracefully, no alert needed.
+    if (error instanceof ServiceValidationError) {
+      logger.warn("Stream batch items error", { batchId, error: error.message });
+      return json({ error: error.message }, { status: 422 });
+    }
+
+    if (error instanceof Error && error.message.includes("Invalid JSON")) {
+      logger.warn("Stream batch items error: invalid JSON", {
+        batchId,
+        error: error.message,
+      });
+      return json({ error: error.message }, { status: 400 });
+    }
+
     logger.error("Stream batch items error", {
       batchId,
       error: {
@@ -96,14 +112,7 @@ export async function action({ request, params }: ActionFunctionArgs) {
       },
     });
 
-    if (error instanceof ServiceValidationError) {
-      return json({ error: error.message }, { status: 422 });
-    } else if (error instanceof Error) {
-      // Check for stream parsing errors (e.g. invalid JSON)
-      if (error.message.includes("Invalid JSON")) {
-        return json({ error: error.message }, { status: 400 });
-      }
-
+    if (error instanceof Error) {
       return json({ error: error.message }, { status: 500 });
     }