[bugfix] Deref stats async, serve stub collections if handshaking

[tsmethurst]

Description

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This pull request solves a bug introduced in 0.16.0-rc1 where account stats were fetched during the initial dereference attempt, leading to a potential recursive handshake loop where two GtS instances would fail to dereference each other.

The fix is to:

1. Only fetch stats asynchronously, and during a first dereference just return local stats (ie., 0 followers, 0 following, 0 statuses).
2. Serve bare stub endpoints for an Actor's collections (without even totalItems) if dereference is done during handshaking, just to get things moving along. This part isn't strictly necessary to fix this bug now, but it will ensure that future softwares trying to deref GtS collections, who withhold their collections first, can still get something in response.

The PR also stops serving totalItems in cases where the receiving account has decided to hide their collections. This brings it in line with what Mastodon glitch does.

Oh, and the PR also uses our paging code for outbox, in contrast to the sorta freewheeling code we had there before.

Checklist

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• I/we have read the GoToSocial contribution guidelines.
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• I/we have not leveraged AI to create the proposed changes.
• I/we have performed a self-review of added code.
• I/we have written code that is legible and maintainable by others.
• I/we have commented the added code, particularly in hard-to-understand areas.
• I/we have made any necessary changes to documentation.
• I/we have added tests that cover new code.
• I/we have run tests and they pass locally with the changes.
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[NyaaaWhatsUpDoc]

just a few comments but otherwise looking good! nice work 👌