hscontrol/state,db: preserve node expiry on MapRequest updates

Fixes a regression introduced in v0.27.0 where node expiry times were
being reset to zero when tailscaled restarts and sends a MapRequest.

The issue was caused by using GORM's Save() method in persistNodeToDB(),
which overwrites ALL fields including zero values. When a MapRequest
updates a node (without including expiry information), Save() would
overwrite the database expiry field with a zero value.

Changed to use Updates() which only updates non-zero values, preserving
existing database values when struct pointer fields are nil.

In BackfillNodeIPs, we need to explicitly update IPv4/IPv6 fields even
when nil (to remove IPs), so we use Select() to specify those fields.

Added regression test that validates expiry is preserved after MapRequest.

Fixes #2862

hscontrol/db/ip.go

@@ -325,7 +325,11 @@ func (db *HSDatabase) BackfillNodeIPs(i *IPAllocator) ([]string, error) {
 			}
 
 			if changed {
-				err := tx.Save(node).Error
+				// Use Updates() with Select() to only update IP fields, avoiding overwriting
+				// other fields like Expiry. We need Select() because Updates() alone skips
+				// zero values, but we DO want to update IPv4/IPv6 to nil when removing them.
+				// See issue #2862.
+				err := tx.Model(node).Select("ipv4", "ipv6").Updates(node).Error
 				if err != nil {
 					return fmt.Errorf("saving node(%d) after adding IPs: %w", node.ID, err)
 				}