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
* reformat code
This is mostly an automated change with `make lint`.
I had to manually please golangci-lint in routes_test because of a short
variable name.
* fix start -> strategy which was wrongly corrected by linter
