fix: improve analytics accuracy and cost calculation

Refined CalculateCost to correctly handle cached token discounts. Added fuzzy matching to model lookup. Robustified SQL date extraction using SUBSTR and LIKE for better SQLite compatibility.

internal/server/dashboard.go

@@ -254,9 +254,10 @@ func (s *Server) handleUsageSummary(c *gin.Context) {
 			COUNT(*) as today_requests,
 			COALESCE(SUM(cost), 0.0) as today_cost
 		FROM llm_requests
-		WHERE strftime('%Y-%m-%d', timestamp) = ?
-	`, today)
+		WHERE timestamp LIKE ?
+	`, today+"%")
 	if err != nil {
+		fmt.Printf("[ERROR] Failed to fetch today stats: %v\n", err)
 		todayStats.TodayRequests = 0
 		todayStats.TodayCost = 0.0
 	}
@@ -305,7 +306,8 @@ func (s *Server) handleTimeSeries(c *gin.Context) {
 
 	query := fmt.Sprintf(`
 		SELECT 
-			COALESCE(strftime('%%Y-%%m-%%d', timestamp), 'unknown') as bucket,
+			COALESCE(SUBSTR(timestamp, 1, 10), 'unknown') as bucket,
+
 			COUNT(*) as requests,
 			COALESCE(SUM(total_tokens), 0) as tokens,
 			COALESCE(SUM(cost), 0.0) as cost
@@ -468,7 +470,7 @@ func (s *Server) handleDetailedUsage(c *gin.Context) {
 
 	query := fmt.Sprintf(`
 		SELECT 
-			COALESCE(strftime('%%Y-%%m-%%d', timestamp), 'unknown') as date,
+			COALESCE(SUBSTR(timestamp, 1, 10), 'unknown') as date,
 			COALESCE(client_id, 'unknown') as client,
 			COALESCE(provider, 'unknown') as provider,
 			COALESCE(model, 'unknown') as model,