feat: add modular services and repositories for improved code organization

Introduce repository pattern across multiple services, including `favorites`, `tags`, `sessions`, `reports`, `auth`, and more. Decouple crypto functionalities into modular services like `HashingService`, `JwtService`, and `EncryptionService`. Improve testability and maintainability by simplifying dependencies and consolidating utility logic.

backend/src/favorites/favorites.module.ts

@@ -2,11 +2,12 @@ import { Module } from "@nestjs/common";
 import { DatabaseModule } from "../database/database.module";
 import { FavoritesController } from "./favorites.controller";
 import { FavoritesService } from "./favorites.service";
+import { FavoritesRepository } from "./repositories/favorites.repository";
 
 @Module({
 	imports: [DatabaseModule],
 	controllers: [FavoritesController],
-	providers: [FavoritesService],
+	providers: [FavoritesService, FavoritesRepository],
 	exports: [FavoritesService],
 })
 export class FavoritesModule {}