Complete foundational data layer with repository implementation

- Add GORM dependencies for SQLite and PostgreSQL
- Create domain models (User, Session, File) with common errors
- Implement repository interfaces and database layer with migrations
- Update WebApp to bootstrap with database and repositories
- Add comprehensive unit tests for repository methods
- Update config structure to support multiple database drivers
- Extend AGENTS.md with debugging principles and dependency rules

internal/model/errors.go

@@ -0,0 +1,10 @@
+package model
+
+import "errors"
+
+var (
+	ErrNotFound     = errors.New("resource not found")
+	ErrDuplicate    = errors.New("resource already exists")
+	ErrUnauthorized = errors.New("unauthorized")
+	ErrForbidden    = errors.New("forbidden")
+)

internal/model/file.go

@@ -0,0 +1,19 @@
+package model
+
+import (
+	"time"
+)
+
+// File represents a file or directory entry in the virtual filesystem.
+type File struct {
+	ID          string    `gorm:"primaryKey;type:varchar(36)" json:"id"`
+	UserID      string    `gorm:"index;type:varchar(36);not null" json:"user_id"`
+	ParentID    *string   `gorm:"index;type:varchar(36)" json:"parent_id"`
+	Name        string    `gorm:"type:varchar(255);not null" json:"name"`
+	Size        int64     `gorm:"default:0" json:"size"`
+	MimeType    string    `gorm:"type:varchar(127)" json:"mime_type"`
+	StoragePath string    `gorm:"type:varchar(512)" json:"storage_path"`
+	IsDir       bool      `gorm:"default:false" json:"is_dir"`
+	CreatedAt   time.Time `json:"created_at"`
+	UpdatedAt   time.Time `json:"updated_at"`
+}

internal/model/session.go

@@ -0,0 +1,14 @@
+package model
+
+import (
+	"time"
+)
+
+// Session stores a refresh token for a user session.
+type Session struct {
+	ID        string    `gorm:"primaryKey;type:varchar(36)" json:"id"`
+	UserID    string    `gorm:"index;type:varchar(36);not null" json:"user_id"`
+	TokenHash string    `gorm:"uniqueIndex;type:varchar(255);not null" json:"-"`
+	ExpiresAt time.Time `gorm:"not null" json:"expires_at"`
+	CreatedAt time.Time `json:"created_at"`
+}

internal/model/user.go

@@ -0,0 +1,16 @@
+package model
+
+import (
+	"time"
+)
+
+// User represents a registered account.
+type User struct {
+	ID           string    `gorm:"primaryKey;type:varchar(36)" json:"id"`
+	Username     string    `gorm:"uniqueIndex;type:varchar(64);not null" json:"username"`
+	Email        string    `gorm:"uniqueIndex;type:varchar(255);not null" json:"email"`
+	PasswordHash string    `gorm:"type:varchar(255);not null" json:"-"`
+	IsAdmin      bool      `gorm:"default:false" json:"is_admin"`
+	CreatedAt    time.Time `json:"created_at"`
+	UpdatedAt    time.Time `json:"updated_at"`
+}