Emit contract events

[Omoboi-dev]

No events are published, so off-chain UIs/indexers can't react. Emit events for `register_policy`, `pay_premium`, `deposit_liquidity`, and `trigger_payout`.

Acceptance criteria

• Each state-changing fn publishes a structured event.
• Tests assert events via `env.events()`.

[Jaydbrown]

I would like to work on this.