Fix data races, nil-record panics, scanner SSRF, log date layout, shutdown timeouts, and enable stricter linters (#54)

High (behavioral bugs):
- service.Geo: add RLock on lookups, handle Reload error, keep old DBs on failure
- models.GeoDB.Reload: open new DBs before closing old ones (no reader gap)
- router: guard nil geo records (404 instead of 500) in geo.go, generic.go, dns.go
- router/port_scanner: reject undetermined client IPs (nil->localhost dial)
- internal/httputils: fix access log date layout (02/Jan/2006, not literal Nov)
- server/quic: log.Printf instead of log.Fatal in request handler

Medium:
- server/*: 10s shutdown timeout; SIGTERM drains servers before closing geo DBs
- httputils: Clone() request headers instead of mutating the live map; strings.Builder
- setting/app: handle all os.Stat errors; fix "truster" typos; use errors.New;
  remove dead Token field
- cmd: errors.Is for sentinel checks; os.Stderr; log.Fatal instead of panic;
  pass http.Handler by value (not pointer-to-interface)
- resolver: validate config at startup (pre-parse RRs, validate IPs); check WriteMsg
  errors; Setup returns error

Low:
- .golangci.yaml: enable errcheck, govet, errorlint; fix all resulting findings
- server/tls: explicit TLSConfig{MinVersion: tls.VersionTLS12}
- router/dns: label-exact vhost suffix check, strip port; template.Must
- models: errors.Join instead of %s on []error
- service/geo_test: race-regression test (concurrent lookups during Reload)
- README: Go >= 1.25; fix latent test bug (store.Add->Set in dns_test.go)

server/server.go

@@ -5,10 +5,13 @@ import (
 	"os"
 	"os/signal"
 	"syscall"
+	"time"
 
 	"github.com/dcarrillo/whatismyip/service"
 )
 
+const shutdownTimeout = 10 * time.Second
+
 type Server interface {
 	Start()
 	Stop()
@@ -29,11 +32,10 @@ func Setup(servers []Server, geoSvc *service.Geo) *Manager {
 func (m *Manager) Run() {
 	m.start()
 
-	signalChan := make(chan os.Signal, len(m.servers))
+	signalChan := make(chan os.Signal, 1)
 	signal.Notify(signalChan, syscall.SIGHUP, syscall.SIGINT, syscall.SIGTERM)
-	var s os.Signal
 	for {
-		s = <-signalChan
+		s := <-signalChan
 
 		if s == syscall.SIGHUP {
 			m.stop()
@@ -43,10 +45,10 @@ func (m *Manager) Run() {
 			m.start()
 		} else {
 			log.Print("Shutting down...")
+			m.stop()
 			if m.geoSvc != nil {
 				m.geoSvc.Shutdown()
 			}
-			m.stop()
 			break
 		}
 	}