fix(usage): filter totalRequests by selected time period (#857)

The totalRequests on dashboard/usage overview always showed lifetime total
instead of respecting the selected period (24h/7d/30d/60d). Now calculated
from period-filtered data like other stats.

Co-authored-by: Tuan-TC <tuan-tc@users.noreply.github.com>

src/lib/usageDb.js

@@ -533,12 +533,9 @@ export async function getUsageStats(period = "all") {
     })
     .slice(0, 20);
 
-  const lifetimeTotalRequests = typeof db.data.totalRequestsLifetime === "number"
+  // totalRequests: calculated from period-filtered data (not lifetime)
-    ? db.data.totalRequestsLifetime
-    : history.length;
-
   const stats = {
-    totalRequests: lifetimeTotalRequests,
+    totalRequests: 0,
     totalPromptTokens: 0, totalCompletionTokens: 0, totalCost: 0,
     byProvider: {}, byModel: {}, byAccount: {}, byApiKey: {}, byEndpoint: {},
     last10Minutes: [],
@@ -799,6 +796,9 @@ export async function getUsageStats(period = "all") {
     }
   }
 
+  // Calculate totalRequests from period-filtered data (not lifetime)
+  stats.totalRequests = Object.values(stats.byProvider).reduce((sum, p) => sum + (p.requests || 0), 0);
+
   return stats;
 }