PR title: feat(creator-keys): add get_cooldown_remaining view (#1050)

[Cmitchelle7]

Summary

Closes #1050.

The trade cooldown system merged into main covers set_cooldown, get_cooldown_status, per-(creator, wallet) enforcement in buy_key/sell_key, and the CooldownViolationEvent.seconds_remaining payload — verified against creator-keys/src/lib.rs:11221 and :11249.

This PR adds the one piece of the original issue text that isn't yet exposed: get_cooldown_remaining(key_id, wallet) -> u64 — a convenience view returning seconds until the cooldown expires (0 when inactive). Matches the name used in #1050 verbatim.

Changes

creator-keys/src/lib.rs

• get_cooldown_remaining(key_id, wallet) — thin wrapper over get_cooldown_status that returns expires_at - now (0 when inactive or unconfigured)

creator-keys/tests/trade_cooldown.rs

• test_get_cooldown_remaining_tracks_window — full duration → half → 0
• test_get_cooldown_remaining_zero_when_unconfigured — 0 when no cooldown
• test_get_cooldown_remaining_is_per_wallet — one wallet's timer doesn't affect another's

Test plan

• 3 new tests appended to the existing trade_cooldown.rs
• cargo test -p creator-keys trade_cooldown (CI will verify)

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