feat: disable dead subscribers

[wa0x6e]

Subscribers (http and discord) can both die.

• A request to a dead http endpoint will either time out, or reject immediately
• A dead discord (channel removed, permissions issues) will return an error by the discord API

In the case of timeout, connection will be open until timeout, so it's holding a TCP connection open for 30s (or whatever the timeout is set to) open, for nothing.

We should have a strategy to disable dead subscribers.

• Add a status column in the subscribers/subscriptions table
• Everyone has status ALIVE by default
• Add a last_alive column in the subscribers/subscriptions table
• Touch that column on success request
• Each time we send a request, and it fails, we check that column and see for how long it was dead.
• If dead for more than X days, we mark the subscriber as dead (update status to DEAD)
• We send webhook events only to alive subscribers

This strategy will avoid wasting resources on dead subscribers.

[sentry-io]

Sentry issue: SNAPSHOT-WEBHOOK-4

[sentry-io]

Sentry issue: SNAPSHOT-WEBHOOK-3

[sentry-io]

Sentry issue: SNAPSHOT-WEBHOOK-5