SSE single-poller fan-out: bound Redis load to O(queues), not O(connections)

[StrangeNoob]

Summary

Replaces the per-connection SSE polling on GET /api/stream with a single in-process fan-out hub, so dashboard streaming scales with queue count instead of connection count.

• Before: every open dashboard ran its own server-side goroutine polling Redis every ~1s (a SCAN for queue discovery + Stats/Counters per queue). Redis load was O(connections × queues)/sec, and every connection recomputed the identical global snapshot — the limiter well before Go's HTTP layer.
• After: one lazy background poller per process reads Redis once per interval and broadcasts a single snapshot to all subscribers. Redis load is O(queues)/sec, independent of connection count; an idle server (no dashboards) does no polling.

This implements the fan-out that CLAUDE.md previously flagged as future work.

Design

• New hub (internal/api/hub.go) owns all polling + broadcast, behind a snapshotSource interface (satisfied by *broker.Broker) so its fan-out/lifecycle is unit-tested without Redis.
• Lazy lifecycle: poller starts on the first subscribe, stops on the last unsubscribe.
• Latest-wins delivery: each subscriber has a cap-1 channel; a slow tab only ever holds the newest snapshot and never blocks the poller.
• Instant populate preserved: the poller polls immediately on start, and late joiners are seeded from the cached last snapshot.
• Wire format byte-identical (data: <json array>\n\n) — the committed web/dist dashboard is untouched; no frontend change, no dist rebuild.
• Spec: docs/superpowers/specs/2026-06-09-relay-sse-single-poller-fanout-design.md.

Test Plan

• go test -race ./... — all packages pass, including the real-Redis TestStreamEmitsSnapshot (runs through the hub, not skipped)
• Six new hub unit tests (no Redis): lazy start/stop, fan-out to all, slow-consumer-doesn't-block + latest-wins, late-joiner cache, poll-error survival
• go build ./... and go vet ./internal/api/ clean
• web/ untouched; only CLAUDE.md + 4 internal/api files changed
• Spec-compliance, code-quality, and final holistic reviews all passed

[StrangeNoob]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.

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