feat: improve app security and error handling

Improve overall app security by:
- using dynamic statements for all sql querries
- introducing environment variables for initial admin password
- introducing enironment variable for cors address
- improving error handling

backend/models.go

@@ -1,6 +1,9 @@
 package main
 
-import "time"
+import (
+	"github.com/golang-jwt/jwt/v5"
+	"time"
+)
 
 type TimeEntry struct {
 	ID         int       `json:"id"`
@@ -96,4 +99,5 @@ type Claims struct {
 	UserID   int    `json:"user_id"`
 	Username string `json:"username"`
 	IsAdmin  bool   `json:"is_admin"`
+	jwt.RegisteredClaims
 }