refactor visitor IPs and allow exempting IP Ranges

Use netip.Addr instead of storing addresses as strings. This requires
conversions at the database level and in tests, but is more memory
efficient otherwise, and facilitates the following.

Parse rate limit exemptions as netip.Prefix. This allows storing IP
ranges in the exemption list. Regular IP addresses (entered explicitly
or resolved from hostnames) are IPV4/32, denoting a range of one
address.

server/message_cache_test.go

@@ -3,11 +3,13 @@ package server
 import (
 	"database/sql"
 	"fmt"
-	"github.com/stretchr/testify/assert"
-	"github.com/stretchr/testify/require"
+	"net/netip"
 	"path/filepath"
 	"testing"
 	"time"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
 )
 
 func TestSqliteCache_Messages(t *testing.T) {
@@ -281,7 +283,7 @@ func testCacheAttachments(t *testing.T, c *messageCache) {
 	expires1 := time.Now().Add(-4 * time.Hour).Unix()
 	m := newDefaultMessage("mytopic", "flower for you")
 	m.ID = "m1"
-	m.Sender = "1.2.3.4"
+	m.Sender = netip.MustParseAddr("1.2.3.4")
 	m.Attachment = &attachment{
 		Name:    "flower.jpg",
 		Type:    "image/jpeg",
@@ -294,7 +296,7 @@ func testCacheAttachments(t *testing.T, c *messageCache) {
 	expires2 := time.Now().Add(2 * time.Hour).Unix() // Future
 	m = newDefaultMessage("mytopic", "sending you a car")
 	m.ID = "m2"
-	m.Sender = "1.2.3.4"
+	m.Sender = netip.MustParseAddr("1.2.3.4")
 	m.Attachment = &attachment{
 		Name:    "car.jpg",
 		Type:    "image/jpeg",
@@ -307,7 +309,7 @@ func testCacheAttachments(t *testing.T, c *messageCache) {
 	expires3 := time.Now().Add(1 * time.Hour).Unix() // Future
 	m = newDefaultMessage("another-topic", "sending you another car")
 	m.ID = "m3"
-	m.Sender = "1.2.3.4"
+	m.Sender = netip.MustParseAddr("1.2.3.4")
 	m.Attachment = &attachment{
 		Name:    "another-car.jpg",
 		Type:    "image/jpeg",