feat(api,web): event-stream timeline (Phase 3)

Adds GET /api/tasks/{id}/events (seq-ordered, ?since_seq for incremental
polling) as the timeline data source, and a per-task Timeline section in the
web UI that fetches and renders the observability event stream. New pure,
unit-tested JS helpers: formatEventText, renderEventTimeline, fetchTaskEvents.

The timeline load is async and guarded on a global fetch so the synchronous
renderTaskPanel path (and its DOM-mock unit tests) is unaffected. Verified via
Go endpoint tests and node --test (272 JS tests pass). Note: the live in-browser
panel render was not exercised in this environment — only unit-level coverage.

https://claude.ai/code/session_01SESwn7kQ7oP62trWw6pc39

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