feat(dashboard): enable webhooks from the Webhooks page (salvage #43999)

[teknium1]

Summary

Salvage of #43999 by @shannonsands — the dashboard Webhooks page can now enable its own receiver in one click instead of sending users to the Channels page.

• New POST /api/webhooks/enable endpoint: flips platforms.webhook.enabled via the existing _write_platform_enabled helper and kicks a deduped gateway restart (_spawn_gateway_restart — reuses any in-flight restart, never spawns a second).
• Webhooks page replaces the "go to Channels" hint with an inline Enable webhooks CTA, restart-outcome polling, and a manual Restart Gateway fallback card if the auto-restart fails.
• The PUT /api/messaging/platforms/webhook toggle path is intentionally left restart-free (bulk channel edits shouldn't each trigger a restart); the dedicated endpoint exists to bundle enable + restart for the one-click UX.

Validation

Check	Result
tests/hermes_cli/test_dashboard_admin_endpoints.py on current main	69/69 pass
eslint src/pages/WebhooksPage.tsx src/lib/api.ts	clean
npm run build --workspace web	green
E2E (temp HERMES_HOME): _write_platform_enabled("webhook", True) → _is_webhook_enabled()	True — write and read paths agree on platforms.webhook.enabled
Auth gating (test_webhooks_enable_post_gated)	401/403 without session token

Cherry-picked with @shannonsands's authorship preserved; merge via rebase. Closes #43999.

Infographic

[github-actions]

🔎 Lint report: hermes/hermes-64b94540 vs origin/main

ruff

Total: 0 on HEAD, 0 on base (➖ 0)

🆕 New issues: none

✅ Fixed issues: none

Unchanged: 0 pre-existing issues carried over.

ty (type checker)

Total: 10675 on HEAD, 10675 on base (➖ 0)

🆕 New issues: none

✅ Fixed issues: none

Unchanged: 5579 pre-existing issues carried over.

Diagnostics are surfaced as warnings — this check never fails the build.