feat(brands): Add margin estimation to stores/performance endpoint

- Add ?margin_pct query param (default 50% industry standard)
- Returns margin_pct and margin_est per store
- Includes margin_pct_assumed in response metadata

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

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

backend/src/routes/brands.ts

@@ -710,6 +710,8 @@ export function createBrandsRouter(pool: Pool): Router {
       const stateCode = req.query.state as string | undefined;
       const limit = parseLimit(req.query.limit as string, 100);
       const offset = parseOffset(req.query.offset as string);
+      // Margin assumption - default 50% (industry standard)
+      const marginPct = Math.min(Math.max(parseFloat(req.query.margin_pct as string) || 50, 0), 100);
 
       const startDate = new Date();
       startDate.setDate(startDate.getDate() - days);
@@ -879,11 +881,15 @@ export function createBrandsRouter(pool: Pool): Router {
         brand: brandName,
         period_days: days,
         state: stateCode || 'all',
+        margin_pct_assumed: marginPct,
         summary: {
           total_stores: parseInt(totals.total_stores) || 0,
           total_oos: parseInt(totals.total_oos) || 0,
         },
-        stores: result.rows.map(row => ({
+        stores: result.rows.map(row => {
+          const totalSalesEst = row.total_sales_est ? parseFloat(row.total_sales_est) : null;
+          const marginEst = totalSalesEst ? totalSalesEst * (marginPct / 100) : null;
+          return {
           store_id: row.store_id,
           store_name: row.store_name,
           state_code: row.state_code,
@@ -896,7 +902,10 @@ export function createBrandsRouter(pool: Pool): Router {
           oos_pct: parseInt(row.oos_pct) || 0,
           // Velocity & Sales
           avg_daily_units: row.avg_daily_units ? parseFloat(row.avg_daily_units) : null,
-          total_sales_est: row.total_sales_est ? parseFloat(row.total_sales_est) : null,
+          total_sales_est: totalSalesEst,
+          // Margin (estimated)
+          margin_pct: marginPct,
+          margin_est: marginEst ? parseFloat(marginEst.toFixed(2)) : null,
           // Inventory
           avg_days_on_hand: row.avg_days_on_hand ? parseFloat(row.avg_days_on_hand) : null,
           total_stock: row.total_stock ? parseInt(row.total_stock) : null,
@@ -906,7 +915,7 @@ export function createBrandsRouter(pool: Pool): Router {
           lost_opportunity: row.lost_opportunity ? parseFloat(row.lost_opportunity) : null,
           // Categories breakdown
           categories: row.categories || {},
-        })),
+        };}),
         pagination: {
           limit,
           offset,