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.

tests/support/pocketbase.ts

@@ -83,6 +83,8 @@ export interface TestCategory {
 
 export interface TestPocketbase {
   url: string
+  /** Superuser auth token, for tests that need to create/inspect records beyond the seeded fixtures. */
+  token: string
   categories: TestCategory[]
   articles: TestArticle[]
   stop(): Promise<void>
@@ -155,6 +157,7 @@ export async function startTestPocketbase(options: StartOptions = {}): Promise<T
 
   return {
     url,
+    token,
     categories,
     articles,
     async stop() {