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/tcp.go

@@ -11,11 +11,11 @@ import (
 
 type TCP struct {
 	server  *http.Server
-	handler *http.Handler
+	handler http.Handler
 	ctx     context.Context
 }
 
-func NewTCPServer(ctx context.Context, handler *http.Handler) *TCP {
+func NewTCPServer(ctx context.Context, handler http.Handler) *TCP {
 	return &TCP{
 		handler: handler,
 		ctx:     ctx,
@@ -25,7 +25,7 @@ func NewTCPServer(ctx context.Context, handler *http.Handler) *TCP {
 func (t *TCP) Start() {
 	t.server = &http.Server{
 		Addr:         setting.App.BindAddress,
-		Handler:      *t.handler,
+		Handler:      t.handler,
 		ReadTimeout:  setting.App.Server.ReadTimeout,
 		WriteTimeout: setting.App.Server.WriteTimeout,
 	}
@@ -40,7 +40,9 @@ func (t *TCP) Start() {
 
 func (t *TCP) Stop() {
 	log.Print("Stopping TCP server...")
-	if err := t.server.Shutdown(t.ctx); err != nil {
+	ctx, cancel := context.WithTimeout(t.ctx, shutdownTimeout)
+	defer cancel()
+	if err := t.server.Shutdown(ctx); err != nil {
 		log.Printf("TCP server forced to shutdown: %s", err)
 	}
 }