fix(mirror): don't cache a suspended empty-upstream placement as mirrored

The probe-cache write-through ran right after CreateMirror, keyed off the response's Suspended flag — but for an existing empty-upstream placement suspension only surfaces via the follow-up GetMirror read, so a suspended placement was cached as Mirrored and the onboarding checklist rendered a green 'Repo mirrored' rung for the cache TTL while the mirror never serves.

Move the write-through (healMirrorProbeCache) after every suspension signal has been checked.

Trail-Finding: 019f5de7-4acf-7bda-aae4-1936c5b7cbaa