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

docs/roadmap.md

@@ -16,12 +16,12 @@
 Package-level implementation order (each task includes unit tests):
 
 1. `internal/config` — Viper loader, config struct
-2. `internal/app` — runtime dependency container 🛠 WIP
-3. `internal/model` — domain types, error codes
-4. `internal/api` — error response helpers 🛠 WIP
+2. `internal/app` — runtime dependency container ✅
+3. `internal/model` — domain types, error codes ✅
+4. `internal/api` — error response helpers ✅
 5. `internal/auth` — JWT utils
 6. `internal/storage` — backend interface + local fs
-7. `internal/repository` — interfaces + GORM/SQLite impl
+7. `internal/repository` — interfaces + GORM/SQLite impl ✅
 8. `internal/service` — auth, file, admin services
 9. `internal/middleware` — logger, cors, auth
 10. `internal/handler` — auth, file, admin handlers 🛠 WIP