[#903] Make leaderboard ordering and cursor pagination deterministic

[mikewheeleer]

Summary

• use user_id as the final tie-breaker when assigning leaderboard ranks and when reading ranked rows
• validate decoded cursor ranks before constructing a keyset predicate
• preserve the look-ahead row in the cached first page so cache hits retain nextCursor
• document the materialized-view snapshot behavior for refreshes

Acceptance criteria

• Equal scores receive deterministic rank ordering
• Leaderboard reads use the same deterministic (rank, user_id) order
• Cursor inputs with invalid ranking keys fail safely
• Cached first pages continue pagination instead of silently truncating the traversal
• Existing offset callers remain supported

Validation

• git diff --check passed
• Runtime test execution was not available in the isolated worktree because dependencies are not installed

Closes #903