feat(profitability): set default period to this_month on omnichannel margin page

apps/backend/src/routes/profitability.ts

@@ -295,7 +295,7 @@ export const profitabilityRoute = new Elysia({ prefix: "/profitability" })
   .get("/omnichannel", async ({ query }) => {
     const search = (query.search || "").trim().toLowerCase();
     const channel = (query.channel || "all").trim().toLowerCase();
-    const dateRange = resolveDateRange(query.period, query.startDate, query.endDate);
+    const dateRange = resolveDateRange(query.period || "this_month", query.startDate, query.endDate);
 
     // 1. Fetch mapped marketplace products with master variants & BOM
     const mpProducts = await prisma.marketplaceProduct.findMany({