feat(keys): build leaderboard endpoint for top creator keys (#941)

[VictorEzenma]

• Implement GET /keys/leaderboard supporting sort_by dimensions: holder_count, volume_24h, volume_7d,
  price_change
• Add Redis caching with 60s TTL per sort dimension and graceful live computation fallback
• Enforce limit parameter validation up to configured maximum (MAX_PAGE_SIZE = 100)
• Handle tied ranks deterministically with secondary sort by creation date (createdAt) and tertiary keyId
  tie-breaker
• Add comprehensive integration test suite covering all sort dimensions, ties, caching, and validation errors

Closes #941

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