fix(clients): let an explicit 0 actually disable PersistentKeepalive

Addresses review feedback on the previous commit.

UpdateInboundClient carries a stored keepalive forward whenever the incoming
one is zero, so the settings JSON and the running peer survive a metadata-only
edit that omits the field. That was a 0 -> 0 no-op while no UI could set a
nonzero value. Now that the client form can, the carry-forward became reachable
in the other direction: a client created at the form's default of 25 could
never be returned to 0, and the hint text shipped to all 13 locales -- "0
disables it" -- described something the backend silently refused. The save even
reported success, because a settings blob that came back byte-identical skips
the transaction entirely.

The zero value cannot carry that distinction, so model.Client.KeepAlive becomes
*int: nil means the field was never sent, &0 means "send no keepalives". The
pointer survives the internal marshal in ClientService.Update, which is where an
explicit 0 was being erased by omitempty before UpdateInboundClient ever saw it.
ClientRecord.KeepAlive stays a plain int -- it is the stored column, where
"unset" has no meaning -- and the conversions bridge the two.

Two tests, both red before this change in the direction they cover: an explicit
0 must reach wg_keep_alive, and an update that omits the field must still leave
a stored 25 alone.

Also adds the output transform every other numeric field in the client form
already has, so a cleared box sends 0 rather than null.

internal/database/model/model.go

@@ -441,8 +441,8 @@ func WireguardPeerFromClient(c Client) map[string]any {
 	if c.PreSharedKey != "" {
 		peer["preSharedKey"] = c.PreSharedKey
 	}
-	if c.KeepAlive > 0 {
-		peer["keepAlive"] = c.KeepAlive
+	if ka := c.KeepAliveSeconds(); ka > 0 {
+		peer["keepAlive"] = ka
 	}
 	return peer
 }
@@ -890,7 +890,7 @@ type Client struct {
 	// before -- fully backward compatible for callers that never set this.
 	AllowedIPsByInbound map[int][]string `json:"allowedIPsByInbound,omitempty"`
 	PreSharedKey        string           `json:"preSharedKey,omitempty"`
-	KeepAlive           int              `json:"keepAlive,omitempty"`
+	KeepAlive           *int             `json:"keepAlive,omitempty"`      // Seconds between PersistentKeepalive packets; 0 sends none, omit to keep the stored value
 	ForwardedPorts      string           `json:"forwardedPorts,omitempty"` // AmneziaWG per-client port-forwarding spec, e.g. "80,443,8000-8100"
 	Secret              string           `json:"secret,omitempty" example:"ee1234567890abcdef1234567890abcd7777772e636c6f7564666c6172652e636f6d"`
 	AdTag               string           `json:"adTag,omitempty" example:"0123456789abcdef0123456789abcdef"`
@@ -1113,6 +1113,18 @@ type Host struct {
 
 func (Host) TableName() string { return "hosts" }
 
+// KeepAliveSeconds is the client's PersistentKeepalive, 0 when unset.
+func (c Client) KeepAliveSeconds() int {
+	if c.KeepAlive == nil {
+		return 0
+	}
+	return *c.KeepAlive
+}
+
+// KeepAlivePtr wraps an explicit PersistentKeepalive, 0 included -- distinct
+// from a nil KeepAlive, which means the field was never sent.
+func KeepAlivePtr(v int) *int { return &v }
+
 func (c *Client) ToRecord() *ClientRecord {
 	rec := &ClientRecord{
 		Email:           c.Email,
@@ -1141,7 +1153,7 @@ func (c *Client) ToRecord() *ClientRecord {
 		PublicKey:      c.PublicKey,
 		AllowedIPs:     strings.Join(c.AllowedIPs, ","),
 		PreSharedKey:   c.PreSharedKey,
-		KeepAlive:      c.KeepAlive,
+		KeepAlive:      c.KeepAliveSeconds(),
 		ForwardedPorts: c.ForwardedPorts,
 		Secret:         c.Secret,
 		AdTag:          c.AdTag,
@@ -1199,7 +1211,7 @@ func (r *ClientRecord) ToClient() *Client {
 		PublicKey:      r.PublicKey,
 		AllowedIPs:     splitWireguardAllowedIPs(r.AllowedIPs),
 		PreSharedKey:   r.PreSharedKey,
-		KeepAlive:      r.KeepAlive,
+		KeepAlive:      KeepAlivePtr(r.KeepAlive),
 		ForwardedPorts: r.ForwardedPorts,
 		Secret:         r.Secret,
 		AdTag:          r.AdTag,