feat: replay boot-time log backlog to new log stream clients

[CodeMaster4711]

Like docker logs, a new log stream client should see everything since container start, not just what's produced after it connects. Previously stream_logs only forwarded live reads to whatever client was currently attached and dropped everything else — boot-time output (docker-entrypoint.sh, nginx notice lines) was gone forever if no client had connected yet, since firecracker microVMs boot in well under a second and the browser tab always attaches after that.

Adds a bounded 64KB ring buffer (VecDeque) that every stdout/stderr chunk is appended to regardless of whether a client is attached. On accept, the full backlog is replayed to the new client before it starts receiving live chunks.

[github-actions]

cargo audit found vulnerabilities:

• RUSTSEC-2026-0204 crossbeam-epoch@0.9.18: Invalid pointer dereference in fmt::Pointer impl for Atomic and Shared when the underlying pointer is invalid
• RUSTSEC-2026-0235 rkyv@0.7.46: Insufficient archive validation can cause out-of-bounds reads in archives containing Rc/Arc
• RUSTSEC-2023-0071 rsa@0.9.10: Marvin Attack: potential key recovery through timing sidechannels