Build federated social network example and tutorial (Next.js + Node.js)

[dahlia]

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

Scenario

A multi-user federated microblog. Visitors sign up for accounts on the instance; each user gets their own ActivityPub actor with its own URI, inbox, and followers. Users compose short text posts (notes), follow remote fediverse accounts, like posts, and read a home timeline of posts from the people they follow.

This goes beyond the existing Creating your own federated microblog tutorial, which is deliberately single-user. The focus of this example is the server patterns that show up when multiple actors share one Fedify application: per-actor key management, scoped inbox/outbox dispatchers, and cross-actor database queries for timelines.

Stack

• Framework: Next.js (App Router)
• Integration package: @fedify/next
• Runtime: Node.js
• ORM: Drizzle ORM
• Database: PostgreSQL

Feature checklist

• Signup / login: minimal username-and-password auth sufficient to distinguish actors; not the focus of the tutorial.
• Per-user actor (Person): WebFinger, per-user persisted key pair, followers and following collections scoped per actor.
• Post composition: short text notes federated to the author's followers as Create(Note). Replies supported via inReplyTo (single level; deep threading is out of scope).
• Edit / delete own posts: Update(Note) and Delete(Note) with Tombstone.
• Follow / unfollow remote or local actors: sends Follow and Undo(Follow).
• Inbox handlers: Follow (auto-accept), Undo(Follow), Create(Note), Update(Note), Delete(Note), Like, Undo(Like).
• Home timeline: posts from the viewer's own account and from the accounts they follow, ordered by published.
• Local timeline: public posts authored on this instance.
• Likes: users can like any visible post; likes federate as Like and Undo(Like).

Deliverables

1. Create fedify-dev/next-social-network under the fedify-dev org. Full runnable Next.js app implementing the feature checklist above.
2. Write docs/tutorial/social-network.md on fedify.dev as a step-by-step walkthrough of building the example from scratch. The tutorial should cover @fedify/next setup in App Router, modelling users and actors in Drizzle, per-actor dispatchers, and timeline queries.
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/next-social-network links back to the tutorial.
5. Tutorial cross-links the Node.js section of the Deployment docs (Best practices for production deployments #689) and the @fedify/next integration docs for running the example in production.
6. Add an entry to CHANGES.md.

Out of scope

• Direct messages (DMs). Scope creep for this example.
• Media uploads and image/video attachments. Covered by Build federated content sharing example and tutorial (Nuxt + Node.js) #693.
• Deep threading (nested replies beyond one level). Rendering and pagination concerns outweigh the federation-learning value.
• Moderation: blocks, mutes, reports, server-level blocks.
• Lists, bookmarks, notifications UI, admin UI.
• Announce (boosts). Deferred to Build federated content sharing example and tutorial (Nuxt + Node.js) #693, which centers on Announce semantics.