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/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"`
+}