Tournament-scoped SSE nudges

[amrtgaber]

Summary

Implements #293 via the issue's preferred design: nudges stay one global stream on the existing single connection, but each nudge's payload now says which tournaments it concerns, so a client watching one tournament can skip refetches another tournament's poll cycle caused. Fully backward compatible — null (or ignoring the field) means "refetch", exactly the pre-#293 behavior.

Plumbing (worker → table → poll loop → SSE frame):

• nudge_versions.tournament_ids — nullable JSON column (expand-only migration 62ef856f3e47, validated on a throwaway Postgres 16; NULL = unscoped, so mixed-revision rollover is safe).
• emit_nudge(..., tournament_ids=…) stores a deduped, sorted scope inside the same transaction as the data write; poll_for_nudges republishes it; the SSE frame payload becomes {"polled_at": …, "tournament_ids": [...] | null}.
• New get_tournament_ids_for_profiles roster helper resolves a profile set to the tournaments rostering them.

Per-tick scoping (emit cadence unchanged — no freshness regression):

• player_stats → tournaments whose rosters contain the polled batch (the batch is the cross-tournament union, so this is every tournament with a linked tracked entrant — tournaments with empty or unlinked rosters stop waking clients).
• recent_matches → tournaments of the profiles whose fetch succeeded this cycle (a failed profile contributed no write).
• live_matches → tournaments of the previous live set ∪ the new one, with the previous set read before the wholesale rewrite — so a tournament whose entrant just left a game still gets its in_match flip.

README's third-party-frontend section documents the payload contract for consumers.

Tests

585 passed. New: roster-helper mapping (shared profile across tournaments, unknown/empty inputs); scoped + unscoped SSE frame rendering; emit→poll→hub round-trip carrying a deduped sorted scope and None (integration, real Postgres); a two-tournament tick test proving the standings nudge carries only the rostered tournament's id. Existing event/stream/tick tests pass unchanged.

Closes #293