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/model/BaseModel.go

@@ -0,0 +1,18 @@
+package model
+
+import (
+	"github.com/google/uuid"
+	"gorm.io/gorm"
+)
+
+type BaseModel struct {
+	ID string `json:"id" gorm:"primaryKey"`
+}
+
+func (base *BaseModel) BeforeCreate(tx *gorm.DB) (err error) {
+	// generate new uuid if ID not created yet
+	if base.ID == "" {
+		base.ID = uuid.New().String()
+	}
+	return
+}

backend/model/Category.go

@@ -1,9 +1,10 @@
 package model
 
 type Category struct {
-	ID            string        `json:"id"`
+	BaseModel
 	Icon          string        `json:"icon"`
 	Title         string        `json:"title"`
-	Subcategories []Subcategory `json:"subcategories"`
+	Subcategories []Subcategory `json:"subcategories" gorm:"foreignKey:CategoryID"`
 	Type          string        `json:"type"`
+	UserID        string
 }

backend/model/Operation.go

@@ -0,0 +1,16 @@
+package model
+
+type Operation struct {
+	BaseModel
+	Title         string  `json:"title"`
+	Amount        float64 `json:"amount"`
+	Note          string  `json:"note,omitempty"`
+	Icon          string  `json:"icon"`
+	Type          string  `json:"type"`
+	Date          string  `json:"date"`
+	UserID        string
+	CategoryID    string
+	SubcategoryID string
+	Category      Category    `gorm:"foreignKey:CategoryID"`
+	Subcategory   Subcategory `gorm:"foreignKey:SubcategoryID"`
+}

backend/model/Subcategory.go

@@ -0,0 +1,8 @@
+package model
+
+type Subcategory struct {
+	BaseModel
+	Title      string `json:"title"`
+	CategoryID string
+	UserID     string
+}

backend/model/Token.go

@@ -0,0 +1,5 @@
+package model
+
+type TokenRequest struct {
+	Token string `json:"token"`
+}

backend/model/User.go

@@ -0,0 +1,26 @@
+package model
+
+type User struct {
+	BaseModel
+	Username      string        `json:"username" gorm:"unique"`
+	Email         string        `json:"email" gorm:"unique"`
+	Password      string        `json:"password"`
+	Group         string        `json:"group"`
+	IsSuperuser   bool          `json:"is_superuser"`
+	Categories    []Category    `json:"categories" gorm:"foreignKey:UserID"`
+	Subcategories []Subcategory `json:"subcategories" gorm:"foreignKey:UserID"`
+	Operations    []Operation   `json:"operations" gorm:"foreignKey:UserID"`
+}
+
+type AuthRequest struct {
+	Username string `json:"username"`
+	Password string `json:"password"`
+}
+
+type UserResponse struct {
+	ID          string `json:"id"`
+	Username    string `json:"username"`
+	Email       string `json:"email"`
+	Group       string `json:"group"`
+	IsSuperuser bool   `json:"is_superuser"`
+}

backend/model/operation.go

@@ -1,13 +0,0 @@
-package model
-
-type Operation struct {
-	ID          string      `json:"id"`
-	Title       string      `json:"title"`
-	Amount      float64     `json:"amount"`
-	Category    string      `json:"category"`
-	SubCategory Subcategory `json:"subcategory,omitzero"`
-	Note        string      `json:"note,omitempty"`
-	Icon        string      `json:"icon"`
-	Type        string      `json:"type"`
-	Date        string      `json:"date"`
-}

backend/model/subcategory.go

@@ -1,6 +0,0 @@
-package model
-
-type Subcategory struct {
-	ID    string `json:"id"`
-	Title string `json:"title"`
-}

backend/model/user.go

@@ -1,14 +0,0 @@
-package model
-
-type AuthRequest struct {
-	Username string `json:"username"`
-	Password string `json:"password"`
-}
-
-type User struct {
-	Username    string `json:"username"`
-	Password    string `json:"password"`
-	Email       string `json:"email"`
-	Group       string `json:"group"`
-	IsSuperuser bool   `json:"is_superuser"`
-}