feat: allow blog posts to be organized by subdirectories

Blog posts can now be structured into subdirectories, which determine their slugs.

Example:
- `/src/data/blog/2025/testing.md` → `/posts/2025/testing`

Directories prefixed with `_` will be excluded from the slug.

Example:
- `/src/data/blog/_examples/tailwind-typography.md` → `/posts/tailwind-typography`

Closes #470, #366

src/pages/posts/[...slug]/index.png.ts

@@ -0,0 +1,36 @@
+import type { APIRoute } from "astro";
+import { getCollection, type CollectionEntry } from "astro:content";
+import { getPath } from "@/utils/getPath";
+import { generateOgImageForPost } from "@/utils/generateOgImages";
+import { SITE } from "@/config";
+
+export async function getStaticPaths() {
+  if (!SITE.dynamicOgImage) {
+    return [];
+  }
+
+  const posts = await getCollection("blog").then(p =>
+    p.filter(({ data }) => !data.draft && !data.ogImage)
+  );
+
+  return posts.map(post => ({
+    params: { slug: getPath(post.id, post.filePath, false) },
+    props: post,
+  }));
+}
+
+export const GET: APIRoute = async ({ props }) => {
+  if (!SITE.dynamicOgImage) {
+    return new Response(null, {
+      status: 404,
+      statusText: "Not found",
+    });
+  }
+
+  return new Response(
+    await generateOgImageForPost(props as CollectionEntry<"blog">),
+    {
+      headers: { "Content-Type": "image/png" },
+    }
+  );
+};