Update payload.config.ts to assert non-null environment variables

Use non-null assertion operator for R2 environment variables to ensure
they are defined at runtime.

src/payload.config.ts

@@ -72,12 +72,12 @@ export default buildConfig({
       collections: {
         media: true,
       },
-      bucket: process.env.R2_BUCKET,
+      bucket: process.env.R2_BUCKET!,
       config: {
-        endpoint: `https://${process.env.R2_ACCOUNT_ID}.r2.cloudflarestorage.com`,
+        endpoint: `https://${process.env.R2_ACCOUNT_ID!}.r2.cloudflarestorage.com`,
         credentials: {
-          accessKeyId: process.env.R2_ACCESS_KEY_ID,
+          accessKeyId: process.env.R2_ACCESS_KEY_ID!,
-          secretAccessKey: process.env.R2_SECRET_ACCESS_KEY,
+          secretAccessKey: process.env.R2_SECRET_ACCESS_KEY!,
         },
         region: 'auto',
         // ... Other S3 configuration