Reliable event delivery and recovery #332

SubscribeEvents currently uses an in-memory broadcast channel. Subscribers can miss events when disconnected or when they fall behind, and cannot resume from a cursor.

LDK Node persists its event queue and returns the same event until event_handled() is called. LDK Server can use this existing behavior to durably record API events before acknowledging them to LDK Node, then let subscribers replay recorded events after reconnecting.

Proposed behavior

• Convert each LDK Node event into its API envelope and persist it in an ldk-server-owned event journal.
• Acknowledge the event to LDK Node only after the journal write succeeds.
• Assign stable event IDs and allow subscribers to resume from a cursor.
• Define retention, duplicate handling, and behavior for slow subscribers.