Push notifications (RFC 8620 §7)

[SashankBhamidi]

The client polls for changes via get_sync_token. JMAP push subscriptions eliminate that.

Add:

subscribe_push(callback_url, types=None) -> str   # subscription ID
unsubscribe_push(subscription_id)
renew_push(subscription_id)   # subscriptions expire, RFC 8620 §7.2

types defaults to ["CalendarEvent"]. New _methods/push.py with build_push_subscription_set and parse_push_subscription_set.

The callback URL must be HTTPS and publicly reachable by the server. This won't work behind localhost without tunneling. Document that clearly.

• Unit tests with mocked responses
• Integration test skipped in CI (needs public endpoint), document how to run manually