feat: real‑time webhook notifications for Tx / event updates

[qvkare]

Problem

Users have to poll transactions_getStatus; this is slow and wastes RPC calls.

Background

Alchemy Notify and similar services push JSON payloads to a user‑supplied URL on new blocks, tx confirmations, or specific events.

Proposed Solution

1. webhookRegister(url, eventFilter) – returns hookId.
2. webhookDelete(hookId) – deregister.
3. Internal dispatcher (src/webhook/dispatcher.ts) with Express POST /notify.
4. Supports: txConfirmed, erc20Transfer(address), newBlock.

Acceptance Criteria

• End‑to‑end test: create hook → transfer USDC → receive POST in <30 s.
• Hooks survive server restart (use LiteFS or JSON DB).
• Rate‑limit to 5 events / sec per hook.

Labels

enhancement feature:webhook