Add SSH check period tracking so that recently authenticated users
are auto-approved without requiring manual intervention each time.
Introduce SSHCheckPeriod type with validation (min 1m, max 168h,
"always" for every request) and encode the compiled check period
as URL query parameters in the HoldAndDelegate URL.
The SSHActionHandler checks recorded auth times before creating a
new HoldAndDelegate flow. Auth timestamps are stored in-memory:
- Default period (no explicit checkPeriod): auth covers any
destination, keyed by source node with Dst=0 sentinel
- Explicit period: auth covers only that specific destination,
keyed by (source, destination) pair
Auth times are cleared on policy changes.
Updates #1850
