fix: make schema(s) strict (#23)

If schema(s) are made with strict, it will disallow extra keys in the markdown frontmatter.

src/content/_schemas.ts

@@ -1,6 +1,7 @@
 import { z } from "astro:content";
 
-export const blogSchema = z.object({
+export const blogSchema = z
+  .object({
     author: z.string().optional(),
     pubDatetime: z.date(),
     title: z.string(),
@@ -10,6 +11,7 @@ export const blogSchema = z.object({
     tags: z.array(z.string()).default(["others"]),
     ogImage: z.string().optional(),
     description: z.string(),
-});
+  })
+  .strict();
 
 export type BlogFrontmatter = z.infer<typeof blogSchema>;