feat: implement ContentsModule with controllers, services, and DTOs

Added a new ContentsModule to handle content creation, upload, and management. Includes controller endpoints for CRUD operations, content exploration, and tagging. Integrated caching, file processing, S3 storage, and database logic.

backend/src/contents/dto/create-content.dto.ts

@@ -0,0 +1,43 @@
+import {
+	IsArray,
+	IsEnum,
+	IsInt,
+	IsNotEmpty,
+	IsOptional,
+	IsString,
+	IsUUID,
+} from "class-validator";
+
+export enum ContentType {
+	MEME = "meme",
+	GIF = "gif",
+}
+
+export class CreateContentDto {
+	@IsEnum(ContentType)
+	type!: "meme" | "gif";
+
+	@IsString()
+	@IsNotEmpty()
+	title!: string;
+
+	@IsString()
+	@IsNotEmpty()
+	storageKey!: string;
+
+	@IsString()
+	@IsNotEmpty()
+	mimeType!: string;
+
+	@IsInt()
+	fileSize!: number;
+
+	@IsOptional()
+	@IsUUID()
+	categoryId?: string;
+
+	@IsOptional()
+	@IsArray()
+	@IsString({ each: true })
+	tags?: string[];
+}