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)

integration-tests/integration_test.go

@@ -142,7 +142,11 @@ func TestContainerIntegration(t *testing.T) {
 		Started: true,
 	})
 	require.NoError(t, err)
-	t.Cleanup(func() { c.Terminate(ctx) })
+	t.Cleanup(func() {
+		if err := c.Terminate(ctx); err != nil {
+			t.Logf("Failed to terminate container: %s", err)
+		}
+	})
 
 	http.DefaultTransport.(*http.Transport).TLSClientConfig = &tls.Config{InsecureSkipVerify: true}
 	tests := []struct {
@@ -301,7 +305,11 @@ func TestContainerIntegrationDisableScan(t *testing.T) {
 		Started: true,
 	})
 	require.NoError(t, err)
-	t.Cleanup(func() { c.Terminate(ctx) })
+	t.Cleanup(func() {
+		if err := c.Terminate(ctx); err != nil {
+			t.Logf("Failed to terminate container: %s", err)
+		}
+	})
 
 	t.Run("RequestScanEndpointWithDisabledScan", func(t *testing.T) {
 		req, err := http.NewRequest("GET", "http://localhost:8000/scan/tcp/8000", nil)