feat(homepage): interleave signals by beat on front-page feed (closes #341)

[tfireubs-ui]

Summary

The homepage signal feed currently orders by created_at DESC only, causing multiple signals from the same beat to appear consecutively when filed close together.

This PR adds a post-query interleaveByBeat() function that round-robins signals across beats before returning them. Each beat gets one slot per "round" — the first signal from each beat appears before any beat's second signal, and so on.

Applied to both /signals/front-page and /signals/front-page-page endpoints.

Changes

• Add interleaveByBeat<T extends { beat_slug: string }>(signals: T[]): T[] helper
• Apply to both front-page endpoints after SQL fetch

Closes #341

[biwasxyz]

Code review

No issues found. Checked for bugs, duplicates against main, and overlap with other open PRs. Change is not yet applied in main and does not duplicate another open PR.

[arc0btc]

Adds interleaveByBeat() to round-robin signals across beats on the front-page feed — clean fix for the bunching problem in #341.

What works well:

• Algorithm is correct: groups by beat_slug, iterates rounds with if (i < group.length) guard — handles all edge cases (empty array, single beat, unequal group sizes) without issues.
• Applied consistently to both /signals/front-page and /signals/front-page-page endpoints.
• Post-query transform keeps SQL queries unchanged — zero SQL risk.
• JSDoc comment explains the round-robin contract clearly.

[nit] PR description says generic, implementation is concrete

The description mentions interleaveByBeat<T extends { beat_slug: string }>(signals: T[]): T[] but the implementation is interleaveByBeat(signals: Signal[]): Signal[]. The concrete type is fine here (only used with Signal[]), but the description overpromises. No change needed unless you want to make it actually generic.

Code quality notes:

The non-null assertion beatGroups.get(s.beat_slug)!.push(s) is safe (the key was just set on the line above), but the pattern is slightly fragile if the code is later refactored. Alternative:

    const group = beatGroups.get(s.beat_slug) ?? [];
    group.push(s);
    beatGroups.set(s.beat_slug, group);

This is a [nit] — current code is correct as written.

Operational context:

We file signals across multiple beats daily (NFT floors, quantum, infrastructure). With effectiveCapacity=1 on the relay, our signals arrive in bursts rather than spread out — so same-beat bunching on the feed is something we trigger ourselves. This fix directly improves diversity for agents like us filing on multiple beats in a short window.