feat: add env schema for environment variable validation

Introduced `env.schema.ts` for structured validation of environment variables using Zod. Includes defaults and validations for database, S3, security, mail, Redis, and session configurations.

backend/src/config/env.schema.ts

@@ -0,0 +1,63 @@
+import { z } from "zod";
+
+export const envSchema = z.object({
+	NODE_ENV: z.enum(["development", "production", "test"]).default("development"),
+	PORT: z.coerce.number().default(3000),
+
+	// Database
+	POSTGRES_HOST: z.string(),
+	POSTGRES_PORT: z.coerce.number().default(5432),
+	POSTGRES_DB: z.string(),
+	POSTGRES_USER: z.string(),
+	POSTGRES_PASSWORD: z.string(),
+
+	// S3
+	S3_ENDPOINT: z.string().default("localhost"),
+	S3_PORT: z.coerce.number().default(9000),
+	S3_USE_SSL: z.preprocess((val) => val === "true", z.boolean()).default(false),
+	S3_ACCESS_KEY: z.string().default("minioadmin"),
+	S3_SECRET_KEY: z.string().default("minioadmin"),
+	S3_BUCKET_NAME: z.string().default("memegoat"),
+
+	// Security
+	JWT_SECRET: z.string().min(32),
+	ENCRYPTION_KEY: z.string().length(32),
+	PGP_ENCRYPTION_KEY: z.string().min(16),
+
+	// Mail
+	MAIL_HOST: z.string(),
+	MAIL_PORT: z.coerce.number(),
+	MAIL_SECURE: z.preprocess((val) => val === "true", z.boolean()).default(false),
+	MAIL_USER: z.string(),
+	MAIL_PASS: z.string(),
+	MAIL_FROM: z.string().email(),
+
+	DOMAIN_NAME: z.string(),
+	
+	// Sentry
+	SENTRY_DSN: z.string().optional(),
+
+	// Redis
+	REDIS_HOST: z.string().default("localhost"),
+	REDIS_PORT: z.coerce.number().default(6379),
+
+	// Session
+	SESSION_PASSWORD: z.string().min(32),
+
+	// Media Limits
+	MAX_IMAGE_SIZE_KB: z.coerce.number().default(512),
+	MAX_GIF_SIZE_KB: z.coerce.number().default(1024),
+});
+
+export type Env = z.infer<typeof envSchema>;
+
+export function validateEnv(config: Record<string, unknown>) {
+	const result = envSchema.safeParse(config);
+
+	if (!result.success) {
+		console.error("❌ Invalid environment variables:", result.error.format());
+		throw new Error("Invalid environment variables");
+	}
+
+	return result.data;
+}