Fix/IcingaDBWeb: clean stale Icingaweb2 cookie only when Keycloak session is expired

[Plattenspatz]

Fixes #1163

In an IcingaDBWeb + Keycloak setup, Nagstamon can get stuck in a redirect-to-login loop when a persisted Icingaweb2 session cookie from an older run is reused after the Keycloak session already expired server-side.

This PR adds a minimal, server-type-specific cleanup for IcingaDBWeb:

• run cleanup only once per application start / server instance
• only if Keycloak cookies are present
• only if there is no valid KEYCLOAK_SESSION cookie (based on the local expiration timestamp)
• then remove the persisted Icingaweb2 cookie from cookies.db

Additionally, it guards _set_recheck() against missing HTML form elements to avoid a crash when authentication expired and the expected form cannot be found.

[HenriWahl]

Thanks @Plattenspatz!