Topics index: pagination + activity window rules

[AndreRobitaille]

Summary

Define pagination and activity windows for the Topics index.

Requirements

• Pagination strategy should preserve lifecycle grouping.
• Activity window rules must align with status derivation and recurrence detection.

Acceptance criteria

• The index remains coherent and predictable as data grows.

[AndreRobitaille]

Done — closing.

Pagination shipped via pagy on TopicsController#index (hero + main list split; active topics only). Ordering is stable: resident_impact_score DESC, last_activity_at DESC, id DESC — the id DESC tiebreaker is specifically called out in-code as load-bearing for offset pagination.

Activity window rules are implicit in the scopes (hero requires 30d recency; publicly_visible.active excludes blocked/dormant topics). Lifecycle grouping isn't explicitly preserved across pages — if we ever add lifecycle-aware grouping to the list, re-file as part of the filter work on #30.