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/sessions/sessions.module.ts

@@ -2,10 +2,11 @@ import { Module } from "@nestjs/common";
 import { CryptoModule } from "../crypto/crypto.module";
 import { DatabaseModule } from "../database/database.module";
 import { SessionsService } from "./sessions.service";
+import { SessionsRepository } from "./repositories/sessions.repository";
 
 @Module({
 	imports: [DatabaseModule, CryptoModule],
-	providers: [SessionsService],
+	providers: [SessionsService, SessionsRepository],
 	exports: [SessionsService],
 })
 export class SessionsModule {}