feat: implement link upsert API endpoint

- Adds a new API endpoint for creating and retrieving links
- Validates link data using LinkSchema
- Supports case sensitivity for link slugs
- Handles existing links by returning the short link and status
- Creates new links with expiration metadata in Cloudflare KV store

server/api/link/upsert.post.ts

@@ -0,0 +1,38 @@
+import { LinkSchema } from '@/schemas/link'
+
+export default eventHandler(async (event) => {
+  const link = await readValidatedBody(event, LinkSchema.parse)
+  const { caseSensitive } = useRuntimeConfig(event)
+
+  if (!caseSensitive) {
+    link.slug = link.slug.toLowerCase()
+  }
+
+  const { cloudflare } = event.context
+  const { KV } = cloudflare.env
+
+  // Check if link exists
+  const existingLink = await KV.get(`link:${link.slug}`, { type: 'json' })
+
+  if (existingLink) {
+    // If link exists, return it along with the short link
+    const shortLink = `${getRequestProtocol(event)}://${getRequestHost(event)}/${link.slug}`
+    return { link: existingLink, shortLink, status: 'existing' }
+  }
+
+  // If link doesn't exist, create it
+  const expiration = getExpiration(event, link.expiration)
+
+  await KV.put(`link:${link.slug}`, JSON.stringify(link), {
+    expiration,
+    metadata: {
+      expiration,
+      url: link.url,
+      comment: link.comment,
+    },
+  })
+
+  setResponseStatus(event, 201)
+  const shortLink = `${getRequestProtocol(event)}://${getRequestHost(event)}/${link.slug}`
+  return { link, shortLink, status: 'created' }
+})