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)

router/port_scanner.go

@@ -29,8 +29,16 @@ func scanTCPPort(ctx *gin.Context) {
 		return
 	}
 
+	ip := net.ParseIP(ctx.ClientIP())
+	if ip == nil {
+		ctx.JSON(http.StatusBadRequest, JSONScanResponse{
+			Reason: "client ip could not be determined",
+		})
+		return
+	}
+
 	add := net.TCPAddr{
-		IP:   net.ParseIP(ctx.ClientIP()),
+		IP:   ip,
 		Port: port,
 	}