Check notification channel availability for a recipient

Every channel/provider pair this deployment knows, and whether an alert to this recipient could go out over it right now.

Read scope before fixing anything. An unavailable pair fails for one of two reasons, and they are repaired by different people in different places. scope: "recipient" means the person is the blocker — no contact of the required type, the contact is switched off, or the recipient itself is inactive; the account owner fixes that on the recipient. scope: "installation" means the deployment is the blocker — the mailer is not configured (email_not_configured), provider credentials are missing (custom_asterisk_not_configured, sms_not_configured), or telephony is unreachable (asterisk_connection_failed); the recipient's contacts may be perfectly fine and editing them changes nothing. email_not_configured in particular is a property of the installation, never of the person.

reason is a stable machine-readable code; new codes appear as channels are added, so treat an unrecognised one as "cannot fix automatically" and fall back on scope.