fix(webhook): add observability for Gmail history pagination

Log a warning when Gmail API returns nextPageToken, indicating more
history items exist beyond the 500 fetched. This provides visibility
into potential data loss without changing behavior.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

apps/web/app/api/google/webhook/process-history.ts

@@ -326,6 +326,14 @@ async function fetchGmailHistoryResilient({
       historyTypes: ["messageAdded", "labelAdded", "labelRemoved"],
       maxResults: 500,
     });
+
+    if (data.nextPageToken) {
+      logger.warn("Gmail history has more pages that were not fetched", {
+        historyItemCount: data.history?.length ?? 0,
+        startHistoryId,
+      });
+    }
+
     return { status: "success", data, startHistoryId };
   } catch (error) {
     // Gmail history IDs are typically valid for ~1 week. If older, Gmail returns a 404.