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/crypto/services/jwt.service.ts

@@ -0,0 +1,37 @@
+import { Injectable, Logger } from "@nestjs/common";
+import { ConfigService } from "@nestjs/config";
+import * as jose from "jose";
+
+@Injectable()
+export class JwtService {
+	private readonly logger = new Logger(JwtService.name);
+	private readonly jwtSecret: Uint8Array;
+
+	constructor(private configService: ConfigService) {
+		const secret = this.configService.get<string>("JWT_SECRET");
+		if (!secret) {
+			this.logger.warn(
+				"JWT_SECRET is not defined, using a default insecure secret for development",
+			);
+		}
+		this.jwtSecret = new TextEncoder().encode(
+			secret || "default-secret-change-me-in-production",
+		);
+	}
+
+	async generateJwt(
+		payload: jose.JWTPayload,
+		expiresIn = "2h",
+	): Promise<string> {
+		return new jose.SignJWT(payload)
+			.setProtectedHeader({ alg: "HS256" })
+			.setIssuedAt()
+			.setExpirationTime(expiresIn)
+			.sign(this.jwtSecret);
+	}
+
+	async verifyJwt<T extends jose.JWTPayload>(token: string): Promise<T> {
+		const { payload } = await jose.jwtVerify(token, this.jwtSecret);
+		return payload as T;
+	}
+}