feat(auth): Implement database-backed user authentication and

persistence

This commit introduces a major architectural change by replacing the
previous hardcoded user and data system with a robust, database-driven
approach. It lays the foundation for persistent data storage for all
application models.

Key Changes:

- **Database Integration:** - Added `GORM` and `SQLite` for the database
layer. - The database is now initialized and migrated on application
startup. - Configuration is managed through environment variables
(`DATABASE`, `USERS_FILE`).

- **User Authentication & Management:** - The login endpoint
(`/api/token`) now authenticates users against the database. - JWT
claims are now populated with real user data (`id`, `username`,
`is_superuser`). - Implemented a system to seed the database with
initial users from a `users.json` file.

- **New User API Endpoints:** - `GET /api/users`: Retrieves a list of
all users (superuser access required). - `GET /api/users/user`: Fetches
the data for the currently authenticated user based on their JWT.

- **Data Model Overhaul:** - Refactored all data models (`User`,
`Category`, `Operation`, `Subcategory`) to include GORM tags,
relationships, and a `BaseModel` with auto-generated UUIDs for primary
keys. - Separated user-related structs into `User` (database model),
`AuthRequest` (login payload), and `UserResponse` (safe API response).

- **Middleware & Routing:** - Improved JWT error handling for clearer
client-side messages. - Added a new middleware to allow passing the JWT
in the request body for more flexible client integration.

backend/api/GetUserData.go

@@ -0,0 +1,34 @@
+package api
+
+import (
+	"backend/database"
+	"backend/model"
+
+	"github.com/gofiber/fiber/v2"
+	"github.com/golang-jwt/jwt/v5"
+)
+
+func GetUserData(c *fiber.Ctx) error {
+	user := c.Locals("user").(*jwt.Token)
+	claims := user.Claims.(jwt.MapClaims)
+	userid := claims["id"].(string)
+
+	dbuser, e := database.GetUserById(userid)
+	if e != nil {
+		return c.Status(
+			fiber.StatusInternalServerError,
+		).JSON(
+			fiber.Map{
+				"error": e.Error(),
+			},
+		)
+	}
+
+	return c.JSON(model.UserResponse{
+		ID:          dbuser.ID,
+		Username:    dbuser.Username,
+		Email:       dbuser.Email,
+		Group:       dbuser.Group,
+		IsSuperuser: dbuser.IsSuperuser,
+	})
+}