refactor: define blog schema and clean up config

src/content/config.ts

@@ -1,17 +1,8 @@
-import { z, defineCollection } from "astro:content";
+import { defineCollection } from "astro:content";
+import { blogSchema } from "./schemas";
 
 const blogCollection = defineCollection({
-  schema: z.object({
-    author: z.string(),
-    datetime: z.string(), // z.string().datetime() only available in Zod v3.20.2
-    title: z.string(),
-    slug: z.string().optional(),
-    featured: z.boolean(),
-    draft: z.boolean(),
-    tags: z.array(z.string()),
-    ogImage: z.string().optional(),
-    description: z.string(),
-  }),
+  schema: blogSchema,
 });
 
 export const collections = {