Adding a native AI Search plugin, improving search in the cloudflare demo

[ttmx]

Currently our search solution is limited to FTS5 inside an sqlite database.
This does not handle cases where the user's query does not match the literal words.
There may be a post titled "How the 2026 World Cup affected Internet traffic", but this would not be matched by a user query of "sports at cloudflare".

To fix this we should introduce an optional plugin allowing semantic and text search, through Cloudflare's AI Search.

This plugin should automatically sync published posts to an AI Search bucket, and remove them when they are deleted or unpublished.
We should also support multiple languages transparently.