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/config/load.go

@@ -13,7 +13,13 @@ func defaults(v *viper.Viper) {
 	v.SetDefault("server.port", 10086)
 
 	v.SetDefault("database.driver", "sqlite3")
-	v.SetDefault("database.path", "data/mygo.db")
+	v.SetDefault("database.sqlite.path", "data/mygo.db")
+	v.SetDefault("database.postgres.host", "localhost")
+	v.SetDefault("database.postgres.port", 5432)
+	v.SetDefault("database.postgres.user", "mygo")
+	v.SetDefault("database.postgres.password", "")
+	v.SetDefault("database.postgres.dbname", "mygo")
+	v.SetDefault("database.postgres.sslmode", "disable")
 
 	v.SetDefault("storage.driver", "local")
 	v.SetDefault("storage.local.path", "data/files")