refactor: replace SQLite/GORM with JSON file storage

- Remove GORM and SQLite, use simple JSON file (data/certs.json) for persistence
- CertStore with sync.RWMutex for thread-safe in-memory cache
- All handlers updated to use config.Store instead of config.DB
- Cron job and stats updated accordingly
- go mod tidy removes unused gorm/sqlite dependencies

backend/main.go

@@ -16,8 +16,8 @@ import (
 func main() {
 	cfg := config.Load()
 
-	// Initialize database
-	config.InitDB(cfg)
+	// Initialize certificate store (JSON file persistence)
+	config.InitStore(cfg)
 
 	// Setup Gin
 	gin.SetMode(gin.ReleaseMode)
@@ -62,20 +62,19 @@ func main() {
 	c := cron.New()
 	c.AddFunc("0 3 * * *", func() {
 		log.Println("Running scheduled certificate renewal check...")
-		var certs []config.Certificate
-		config.DB.Where("auto_renew = ? AND status = ?", true, "active").Find(&certs)
+		certs := config.Store.GetActiveWithAutoRenew()
 
 		for _, cert := range certs {
 			if cert.ExpiresAt != nil && time.Until(*cert.ExpiresAt).Hours() < float64(cert.RenewDays*24) {
 				log.Printf("Auto-renewing certificate for %s (expires %s)", cert.Domain, cert.ExpiresAt.Format(time.RFC3339))
-				if err := services.RenewCertificate(&cert, cfg); err != nil {
+				if err := services.RenewCertificate(cert, cfg); err != nil {
 					cert.Status = "error"
 					cert.ErrorMessage = "auto renew: " + err.Error()
 					log.Printf("Auto-renew failed for %s: %v", cert.Domain, err)
 				} else {
 					log.Printf("Auto-renew succeeded for %s", cert.Domain)
 				}
-				config.DB.Save(&cert)
+				config.Store.Upsert(cert)
 			}
 		}
 	})