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

config.example.yaml

@@ -4,7 +4,15 @@ server:
 
 database:
     driver: sqlite3
-    path: data/mygo.db
+    sqlite:
+        path: data/mygo.db
+    postgres:
+        host: localhost
+        port: 5432
+        user: mygo
+        password: mygo
+        dbname: mygo
+        sslmode: disable
 
 storage:
     driver: local