Cap articles.content at 10000 characters, with coverage

PocketBase silently enforced its own default max on unbounded text
fields, which broke saving a real production article longer than that.
Make the limit explicit and generous enough for the content this blog
actually publishes, and add a test asserting the exact boundary.

pocketbase/pb_migrations/1757500001_create_articles.js

@@ -18,7 +18,7 @@ migrate((app) => {
     fields: [
       { type: 'text', name: 'title', required: true, max: 160 },
       { type: 'text', name: 'slug', required: true, max: 160, pattern: '^[a-z0-9]+(-[a-z0-9]+)*$' },
-      { type: 'text', name: 'content', required: true },
+      { type: 'text', name: 'content', required: true, max: 10000 },
       {
         type: 'file',
         name: 'cover',