Add Prompt Builder page for multi-scene prompt sequences.

Name, save, and load ordered scenes on the server; assemble copyable text with a Batch-compatible double blank line between scenes.

server/api/prompt-sequences.post.ts

@@ -0,0 +1,19 @@
+import { listPromptSequences, savePromptSequence } from '~/server/utils/promptSequences'
+
+export default defineEventHandler(async (event) => {
+  const { owner } = assertLibraryOwner(event)
+  const body = await readBody<{
+    name?: string
+    scenes?: unknown
+    overwrite?: boolean
+  }>(event).catch(() => ({}))
+  const sequence = await savePromptSequence(owner, {
+    name: body?.name || '',
+    scenes: body?.scenes,
+    overwrite: body?.overwrite === true
+  })
+  return {
+    sequence,
+    sequences: listPromptSequences(owner)
+  }
+})