chore: prepare release 0.3.0

[github-actions]

This PR was created automatically. Merging it will finalize the changelog for 0.3.0

Breaking Changes

Add basic pool support.

Fund one pool per connect key instead of individual accounts. Accounts attached to a pool draw from its shared balance.

Features

• Added reconnecting field to auth connect status so apps can tell if a user is returning or connecting for the first time.

Add network read and write throughput estimates to the client.

The client now exposes ReadEstimate and WriteEstimate, returning the expected duration to transfer a given number of bytes from the network-wide observed throughput. Both fall back to a default rate before any bulk transfers have been sampled.

Fixes

• Chunk RecordIntegrityCheck UPDATEs to bound per-statement latency and shorten row-lock windows.
• Don't renew a contract with capacity 0 if there is another active contract with capacity 0 already
• Rework the unhealthy slabs query to avoid full table scans on large databases.
• Register additional sharing account for each user
• Take into account inflight uploads and downloads when scheduling hosts.

Return a typed HTTPError from the app client on non-2xx responses.

The app client previously returned errors.New("") when an upstream proxy returned a non-2xx status code with an empty body, making failures impossible to diagnose. It now returns an *HTTPError carrying both the status code and body, formatted as HTTP <code>: <message>. Callers can errors.As on it to branch retry behavior on the status code.

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