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/repositories/favorites.repository.ts

@@ -0,0 +1,46 @@
+import { Injectable } from "@nestjs/common";
+import { and, eq } from "drizzle-orm";
+import { DatabaseService } from "../../database/database.service";
+import { contents, favorites } from "../../database/schemas";
+
+@Injectable()
+export class FavoritesRepository {
+	constructor(private readonly databaseService: DatabaseService) {}
+
+	async findContentById(contentId: string) {
+		const result = await this.databaseService.db
+			.select()
+			.from(contents)
+			.where(eq(contents.id, contentId))
+			.limit(1);
+		return result[0] || null;
+	}
+
+	async add(userId: string, contentId: string) {
+		return await this.databaseService.db
+			.insert(favorites)
+			.values({ userId, contentId })
+			.returning();
+	}
+
+	async remove(userId: string, contentId: string) {
+		return await this.databaseService.db
+			.delete(favorites)
+			.where(and(eq(favorites.userId, userId), eq(favorites.contentId, contentId)))
+			.returning();
+	}
+
+	async findByUserId(userId: string, limit: number, offset: number) {
+		const data = await this.databaseService.db
+			.select({
+				content: contents,
+			})
+			.from(favorites)
+			.innerJoin(contents, eq(favorites.contentId, contents.id))
+			.where(eq(favorites.userId, userId))
+			.limit(limit)
+			.offset(offset);
+
+		return data.map((item) => item.content);
+	}
+}