fix(ics): fix ICS subscription key generation if empty

ics secret generation on empty config value was broken since the
config rework. this is fixed in this commit. In addition I added
autogeneration on switch to ics.enable=true.

Also guard EnableSubscription() against file_get_contents() returning
false (e.g. a permission error or the file disappearing after the
file_exists check), so a read failure is treated as a no-op instead of
silently matching against an empty string.

Assisted-by: Claude:claude-sonnet-5