Limit `Bootstrap` payload number of `users`, `posts`

[ok300]

The first time https://pubky.app is opened, it fetches certain data to bootstrap the app, using /v0/bootstrap/{user_id} in the Nexus API.

The same data can be fetched by firing different other calls, like "fetch posts" + "fetch hot tags" + "fetch user details" + "fetch tag counts" etc, but the goal of this endpoint is to avoid this and simplify the bootstrapping of the app.

The issue is described in this TODO:

pubky-nexus/nexus-common/src/models/bootstrap.rs

Line 209 in 4a0163f

// TODO: If the user list is too big, we could do in batches

However there is a better and simpler solution than the proposed batching, namely limiting the max number (in this case of UserViews).

Please address the above TODO by limiting the max number of users and posts a Bootstrap object can contain.

pubky-nexus/nexus-common/src/models/bootstrap.rs

Lines 26 to 35 in 4a0163f

	pub struct Bootstrap {
	/// The user objects shown to the given user ID
	pub users: UserStream,
	/// The posts objects shown to the given user ID
	pub posts: PostStream,
	/// IDs of objects shown to this user on the home page of the FE
	pub ids: BootstrapIds,
	/// Whether or not this user is already indexed
	pub indexed: bool,
	}

The idea is the Bootstrap payload should not be allowed to be that big that a few simple DB lookups cannot handle, but still big enough that it is useful for bootstrapping Pubky App.

To achieve this, introduce some limits, similarly to how it's done for other API endpoints, like

pubky-nexus/nexus-webapi/src/routes/v0/stream/posts.rs

Line 221 in 4a0163f

const MAX_POSTS: usize = 100;

[aintnostressin]

I've only added limit on users. posts are already bounded (20 user's posts + 20x3 replies = 80).