fix: follow-ups from the post-merge reviews of #6221, #6227, #6230 and #6239 (#6250)

* fix: follow-ups from the post-merge reviews of #6221, #6227, #6230 and #6239

Six defects the automated reviews found after those PRs merged. Each is
verified rather than taken on trust — two by experiment, the rest by
reading the merged code.

**Import restore never wrote an empty local value** (#6227). GORM builds
the assignment map from the struct passed to Assign and drops zero-valued
fields, so `Assign(model.Setting{Value: ""})` produced an empty Updates
and the imported row survived. Empty is the normal state: UpdateAllSetting
writes a row for every AllSetting field including the blank ones. That is
exactly the case the PR existed for — a destination with no certificate
inheriting the source machine's path. Confirmed with a throwaway test
before changing anything: the value stayed "IMPORTED". Now uses
saveSetting, which is not zero-filtered.

**Import destroyed node mTLS material** (#6227). The "no local row means
the default applied, so drop the import" branch fires for the five
nodeMtls* keys, which are minted on demand and deliberately absent from
AllSetting, so a fresh install has no row for them. Reinstall-then-restore
therefore deleted the CA certificate and its private key — and the backup
was the only copy, since neither is surfaced in the UI or the export.
Those keys are now kept.

**The clients-list enable toggle wiped renewal state** (#6239, #6238).
setEnable hand-builds the update payload and carried reset but not
resetDay or resetMax, so one click on the switch turned calendar mode off
and lifted the renewal cap permanently. The form-modal tests could not
catch it because that path does send both fields.

**"Delete depleted clients" deleted calendar clients** (#6239). The
predicate read `reset = 0` as "does not auto-renew", which is exactly the
calendar shape, in two places. Both now share one constant that also
requires `reset_day = 0`.

**Allowlist validation and parsing disagreed** (#6230). Save used net,
scan used netip, and they differ: `198.51.100.0/024` saves without
complaint and is silently dropped at scan — the failure the PR set out to
remove. Verified by running both parsers. An IPv4-mapped prefix parsed but
could never match, because contains() unmaps the query while the prefix
stayed 128-bit; it is unmapped at parse now. A test asserts the two
acceptance sets agree.

**A comment stated the opposite of the truth** (#6221). GetInbounds has no
enable filter, so a node reports a disabled inbound normally; the row in
that bug report was missing only because it was never delivered. Reworded
to the real invariant.

Also trims two comment blocks in ip_limit_allowlist.go to the repo's
two-line maximum.

Not included: the reviewer's suggestion to lift the node hand-off out of
`if inbound.Enable` in AddInbound. It is the right root-cause fix, but it
changes delivery behaviour on multi-node deployments and belongs in its
own change with its own testing, not in a cleanup batch.

One reported finding is not real: BulkCreate does call
validateClientResetDay, validateClientResetMax and
validateClientTrafficReset — verified in the merged tree.

* fix(netsafe): wrap both errors so errorlint passes

Unrelated to this PR's subject and in a file it does not otherwise touch.
It is here only because CI lints the merge result, and `main` has been red
since #6242 landed: `fmt.Errorf("%w; %v", ...)` wraps the first error and
formats the second, which errorlint rejects. Go 1.20 allows more than one
%w, so both are wrapped now and `errors.Is` works against either.

internal/web/service/inbound_traffic.go

@@ -21,6 +21,10 @@ import (
 	"gorm.io/gorm/clause"
 )
 
+// A client with a renewal day set auto-renews too, so it must not read as
+// depleted — otherwise the operator's purge deletes it between cycles (#6239).
+const depletedClientsClause = "reset = 0 and reset_day = 0 and ((total > 0 and up + down >= total) or (expiry_time > 0 and expiry_time <= ?))"
+
 func (s *InboundService) AddTraffic(inboundTraffics []*xray.Traffic, clientTraffics []*xray.ClientTraffic) (needRestart bool, clientsDisabled bool, err error) {
 	var disabledNodeIDs []int
 	err = submitTrafficWrite(func() error {
@@ -835,7 +839,7 @@ func (s *InboundService) DelDepletedClients(id int) (err error) {
 		// Collect depleted emails globally — a shared-email row owned by one
 		// inbound depletes every sibling that lists the email.
 		now := time.Now().Unix() * 1000
-		depletedClause := "reset = 0 and ((total > 0 and up + down >= total) or (expiry_time > 0 and expiry_time <= ?))"
+		depletedClause := depletedClientsClause
 		var depletedRows []xray.ClientTraffic
 		if err := tx.Model(xray.ClientTraffic{}).
 			Where(depletedClause, now).