Add last_webhook_update_at timestamp to Subscription model. When a webhook
handler modifies a subscription, it stamps this field. Auto-sync, monitoring,
and force-check services skip subscriptions updated by webhook within the
last 60 seconds, preventing stale panel data from overwriting fresh
real-time changes.
- Add last_webhook_update_at column + migration
- Stamp all 8 webhook handlers with commit in every code path
- Add is_recently_updated_by_webhook() guard in 12 sync/monitoring paths
- Add REMNAWAVE_WEBHOOK_* variables to .env.example
- Add webhook setup documentation to README with Caddy/nginx examples
- Fix pre-existing yookassa webhook test (mock AsyncSessionLocal)
184c52d4ea