Add the Connector Failure Alerts card to admin settings

[rangarajan19]

Summary

PR 2 (UI) for #1006, following up on the merged backend PR #976. Adds the Alerts card to /settings/admin, same shape and conventions as the existing SMTP card right above it.

Changes

• lib/api.ts: adminSettings.getAlertWebhook/updateAlertWebhook/deleteAlertWebhook/testAlertWebhook, typed against the actual merged backend contract (note: GET returns { configured: false } when unset, or the config fields directly with no wrapper key when set — handled by checking for url rather than a configured flag).
• app/settings/admin/page.tsx: new "Connector Failure Alerts" card — URL, payload type (JSON/Slack), threshold/window/cooldown, enabled toggle, save/test/remove, a rotate-secret option, and a shown-once secret reveal using the existing copyText clipboard helper. Shows an https-required hint on Cloud via useAuth().deploymentMode.

Testing

• Tested manually (steps below)
• All existing tests pass (no backend changes in this PR)

This sandbox has no network access for Google Fonts, which breaks next build/next dev under Turbopack (pre-existing on main, confirmed by reproducing it on a clean checkout — unrelated to this change). Built and ran with --webpack instead, which isn't the project's default dev command, so noting it explicitly: npx next build --webpack succeeds, including /settings/admin and the TypeScript pass.

Verified against a real local stack (Postgres via docker compose, migrations applied, backend on :4000, frontend on :3000, webpack mode), logged in as a fresh self-hosted ADMIN, and exercised the full lifecycle on the real merged API:

• Save: generates and reveals the signing secret once, shows the Rotate-secret checkbox and Test/Remove buttons afterward.
• Test: dispatches a real signed POST (hit a real domain without a webhook listener and correctly surfaced Error: Webhook responded with 405, confirming SSRF check → HMAC signing → outbound POST → response status all round-trip into the UI).
• Remove: confirm dialog fires with the expected message, deletes the config, resets the form, shows "Alert webhook removed".

npx tsc --noEmit -p tsconfig.json
npx eslint src/app/settings/admin/page.tsx src/lib/api.ts

Both clean.

Related Issues

Relates to #1006