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/encryption.service.ts

@@ -0,0 +1,58 @@
+import { Injectable, Logger } from "@nestjs/common";
+import { ConfigService } from "@nestjs/config";
+import * as jose from "jose";
+
+@Injectable()
+export class EncryptionService {
+	private readonly logger = new Logger(EncryptionService.name);
+	private readonly jwtSecret: Uint8Array;
+	private readonly encryptionKey: Uint8Array;
+
+	constructor(private configService: ConfigService) {
+		const secret = this.configService.get<string>("JWT_SECRET");
+		this.jwtSecret = new TextEncoder().encode(
+			secret || "default-secret-change-me-in-production",
+		);
+
+		const encKey = this.configService.get<string>("ENCRYPTION_KEY");
+		if (!encKey) {
+			this.logger.warn(
+				"ENCRYPTION_KEY is not defined, using a default insecure key for development",
+			);
+		}
+		const rawKey = encKey || "default-encryption-key-32-chars-";
+		this.encryptionKey = new TextEncoder().encode(
+			rawKey.padEnd(32, "0").substring(0, 32),
+		);
+	}
+
+	async encryptContent(content: string): Promise<string> {
+		const data = new TextEncoder().encode(content);
+		return new jose.CompactEncrypt(data)
+			.setProtectedHeader({ alg: "dir", enc: "A256GCM" })
+			.encrypt(this.encryptionKey);
+	}
+
+	async decryptContent(jwe: string): Promise<string> {
+		const { plaintext } = await jose.compactDecrypt(jwe, this.encryptionKey);
+		return new TextDecoder().decode(plaintext);
+	}
+
+	async signContent(content: string): Promise<string> {
+		const data = new TextEncoder().encode(content);
+		return new jose.CompactSign(data)
+			.setProtectedHeader({ alg: "HS256" })
+			.sign(this.jwtSecret);
+	}
+
+	async verifyContentSignature(jws: string): Promise<string> {
+		const { payload } = await jose.compactVerify(jws, this.jwtSecret);
+		return new TextDecoder().decode(payload);
+	}
+
+	getPgpEncryptionKey(): string {
+		return (
+			this.configService.get<string>("PGP_ENCRYPTION_KEY") || "default-pgp-key"
+		);
+	}
+}