feat: add create-video.sh and four new AI-generated posts

Adds scripts/create-video.sh, which reads a post's prompt.{scene,
environment,style,video} frontmatter and generates its banner video via
Replicate's black-forest-labs/flux-3, extracting the first frame as the
preview PNG. Adds four new posts (the-clockwork-confessor,
the-last-lighthouse, the-ashen-choir, the-serpents-bargain) generated
through this pipeline.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012E2mmywuZyXmPBgSDtNdTL

content/posts/2026-05-28-the-empty-throne/index.md

@@ -3,7 +3,7 @@ title: "The Empty Throne"
 description: "The throne room of the dead — obsidian and black iron, the river Styx visible through the far archway, Cerberus at rest beside the dais — is not empty because Hades is absent. It is empty because he is standing directly behind whoever is looking at it."
 date: 2026-05-28T13:00:00+02:00
 author: "valknar"
-featured: true
+featured: false
 categories:
   - Dark
   - Mythology