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/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"`
+}