Runner: fire-and-forget async event posting with server-side ordering

[nissessenap]

Summary

• Replace synchronous PostEvents in the runner's stdout callback with a fire-and-forget goroutine — prevents blocking the pipe when the API is slow or unreachable
• Add sort.Slice by sequence in EventHub.Publish so out-of-order delivery from concurrent goroutines is corrected server-side
• Add backend test (TestEventHub_PublishOutOfOrder_SortsBySequence) and frontend tests for gap detection and exhausted-retry acceptance

Context

See thoughts/plans/2026-03-01-async-event-posting-comparison.md for the full analysis of fire-and-forget vs buffered channel. Fire-and-forget was chosen for its simplicity — the server-side sort in EventHub eliminates the ordering concern that was the main argument for the channel approach.

Supersedes the buffered channel approach from #37.

Test plan

• TestEventHub_PublishOutOfOrder_SortsBySequence — publishes events as [3, 1, 2], asserts history returns [1, 2, 3]
• TestRunWithEventPosting — updated with thread-safe mock to verify fire-and-forget goroutines complete
• Frontend: out-of-order sequence [1, 3, 2] gap detection and stale skip
• Frontend: exhausted reconnect retries accepts gap and continues
• All existing EventHub, runner, and frontend tests pass
• make lint-fix — 0 issues
• make web-lint-fix, make web-check — clean

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