Fix tournament haystack previous cursor

[yaosongding]

Summary

Fixes a copy-paste bug in TournamentRecordsHaystack where the previous cursor was assigned the value of the next cursor:

// before
tournamentRecordList := &api.TournamentRecordList{Records: results.Records, NextCursor: results.NextCursor, PrevCursor: results.NextCursor, RankCount: results.RankCount}
// after
tournamentRecordList := &api.TournamentRecordList{Records: results.Records, NextCursor: results.NextCursor, PrevCursor: results.PrevCursor, RankCount: results.RankCount}

Impact

Clients paginating backward through ListTournamentRecordsAroundOwner receive a next-page cursor where a previous-page cursor belongs. Following the returned prev_cursor lands on the next page (duplicated forward navigation), and when the leaderboard end leaves no next cursor, no previous cursor is returned at all — backward pagination is broken in both cases.

Evidence

• Introduced in 5666361a ("Add leaderboard record list around owner cursors", Add leaderboard record list around owner cursors #877, 2022) and present since. The sibling path in the same file — TournamentRecordsList (core_tournament.go) — already maps PrevCursor: records.PrevCursor correctly, as does LeaderboardRecordsHaystack (core_leaderboard.go). Only the haystack wrapper dropped it.
• The source value results.PrevCursor has been correct and populated the entire time — only the tournament wrapper dropped it.
• Verified with git history: the line was introduced once and never touched since.

Test

Adds TestApiTournamentHaystack (server/api_tournament_test.go), an API-level test that:

1. Creates a non-authoritative tournament with enable_ranks
2. Writes 5 records via WriteTournamentRecord (scores 10–50, so the owner at score 30 sits at rank 3)
3. Queries ListTournamentRecordsAroundOwner from the middle with a small limit (3)
4. Asserts the middle page contains exactly the records around the owner — scores 40, 30, 20 with ranks 2, 3, 4 — and that PrevCursor != NextCursor
5. Asserts following prev_cursor returns the single record ranked above the middle page (score 50, rank 1), and following next_cursor returns the single record ranked below (score 10, rank 5)
6. Asserts the three pages together cover all five records exactly once

Verified: test FAILS on the buggy code (the cursor-inequality assertion and the previous-page content assertion both fail — the buggy prev_cursor returns the score-10 record instead of the score-50 record), PASSES with the fix.

Verification

• go build ./server/ — passes
• golangci-lint run --new-from-rev=master ./server/ — 0 issues
• New test passes with the fix, fails without it (red-green verified)