[Feature] Contract event monitoring and alerting service

[collinsezedike]

Summary

Nothing consumes the vault's on-chain events. #698 added event emission for admin actions (paused, transfer, accept, adapter, migrate), and #563 tracks extending that to every state-mutating call, but even the events that already exist today have no alerting service watching for them.

Motivation

set_paused, transfer_admin, and migrate_adapter are exactly the actions an operator needs to know about immediately, not discover by manually polling. Without an alerting layer, an unexpected pause, an unauthorized-looking admin rotation, or an adapter migration going through would only be noticed by whoever happens to check the app.

Proposed Solution

A small service (or a scheduled function alongside the existing keepers in packages/stellar-sdk-helpers) that:

• Polls Soroban RPC getEvents for the vault's topic-keyed events (reusing the discovery pattern already built for admin-history.ts)
• Sends an alert (Slack/Discord webhook, or email, whichever the maintainer prefers) on paused, transfer, adapter, and migrate events
• Tracks its own last-processed ledger the same way the accrual/migration keepers track their heartbeat, so a restart doesn't replay old events

Scope

Field	Value
Area	SDK / API
Protocol affected	None
Network	Both
Breaking change?	No

Alternatives Considered

Polling from the frontend was considered and rejected: alerting needs to work even when nobody has the app open, and a browser tab is not a reliable monitoring surface.

Acceptance Criteria

• Service polls vault events on a schedule and dedupes against previously-seen ledgers
• Sends an alert on paused, transfer, adapter, and migrate events
• Documented in the operations docs alongside the existing keeper schedule

Additional Context

Depends on #563 landing further event coverage to be fully useful (deposit/withdraw are not yet instrumented), but the admin-action events already merged in #698 are enough to build the first version against.