Build federated blog example and tutorial (Astro + Bun)

[dahlia]

Sub-issue of #99. Deliver the federated blog scenario as a paired example repository and fedify.dev tutorial, as set in #99 (comment).

Scenario

A single-author federated blog where posts are authored as Markdown/MDX files checked into the repository. Astro builds each post to a static HTML page at build time, while a small set of server routes (the ActivityPub actor, inbox, outbox, and collection endpoints) run in Astro's hybrid SSR mode. This is the workflow most static-blog users already know: git commit → git push → deploy. Federation is a side effect of deploy.

Remote fediverse users can follow the blog and receive new posts in their inboxes; readers from other servers can comment via Note with inReplyTo, and comments are rendered under the post page.

Kept deliberately single-author to distinguish it from the social network example (sibling sub-issue), which is explicitly multi-user.

Stack

• Framework: Astro (hybrid output: static by default, server routes for ActivityPub)
• Content: Astro content collections, Markdown/MDX in src/content/posts/
• Integration package: @fedify/astro
• Runtime: Bun
• Storage: bun:sqlite (Bun's built-in SQLite), no ORM
• Database: SQLite (for followers, comments, queue state, and actor key pair)

Feature checklist

• Blog content lives in src/content/posts/*.md (or .mdx). Each post renders to a static HTML page with a stable canonical URL.
• Actor (Person or Service): WebFinger, persisted key pair (generated once, never regenerated on restart), followers collection.
• Publishing on deploy: for each post in src/content/posts/, compare the current state against SQLite and emit the appropriate activity: Create(Article) for new posts, Update(Article) for edited posts, Delete(Article) (with Tombstone) for removed posts. Idempotent against re-deploys.
• Comments: inbox handlers for Create(Note), Update(Note), and Delete(Note) whose inReplyTo matches a post's canonical URL. Stored in SQLite and rendered under the corresponding post.
• Follow/Undo: inbox handler for Follow (auto-accept) and Undo(Follow).

Deliverables

1. Create fedify-dev/astro-blog under the fedify-dev org. Full runnable Astro app implementing the feature checklist above.
2. Write docs/tutorial/blog.md on fedify.dev as a step-by-step walkthrough of building the example from scratch. The tutorial should cover Astro's hybrid output mode, setting up the content collection, wiring Fedify into server routes, and the publishing-on-deploy pattern.
3. Add the new tutorial to TUTORIAL.items in docs/.vitepress/config.mts (shows up in both top nav and sidebar).
4. README of fedify-dev/astro-blog links back to the tutorial.
5. Tutorial cross-links the Bun and Astro sections of the Deployment docs (Best practices for production deployments #689) for running the example in production.
6. Add an entry to CHANGES.md.

Out of scope

• Multi-author / multi-tenant. Covered by Build federated social network example and tutorial (Next.js + Node.js) #692.
• ORM-based variant. First-pass stack is intentionally raw bun:sqlite.
• Rich authoring UI, image uploads, theming. Keep the example minimal and didactic; authoring is “write a Markdown file and deploy”.