Use storage-aware health probes for Zot registry
Liveness and readiness probes now hit /v2/_catalog?n=1 instead of /v2/,
which actually exercises the storage backend. The shallow /v2/ endpoint
returns 200 even when the JuiceFS mount is stale.
Startup probe threshold bumped to 60×5s to accommodate metadb parsing
over JuiceFS on cold starts.