fix: resolve all golangci-lint issues

Fixes #32

Changes:
- middleware.go: use max() builtin, strconv.Itoa, fix wsl whitespace
- database.go: fix nlreturn, noinlineerr, wsl whitespace
- handlers.go: remove unnecessary template.HTML conversion, unused import
- app.go: extract cleanupContainer to fix nestif, fix lll
- client.go: break long string literals to fix lll
- deploy.go: fix wsl whitespace
- auth_test.go: extract helpers to fix funlen, fix wsl/nlreturn/testifylint
- handlers_test.go: deduplicate IDOR tests, fix paralleltest
- validation_test.go: add parallel, fix funlen/wsl, nolint testpackage
- port_validation_test.go: add parallel, nolint testpackage
- ratelimit_test.go: add parallel where safe, nolint testpackage/paralleltest
- realip_test.go: add parallel, use NewRequestWithContext, fix wsl/funlen
- user.go: (noinlineerr already fixed by database.go pattern)

internal/middleware/realip_test.go

@@ -1,11 +1,14 @@
-package middleware
+package middleware //nolint:testpackage // tests unexported realIP function
 
 import (
+	"context"
 	"net/http"
 	"testing"
 )
 
-func TestRealIP(t *testing.T) {
+func TestRealIP(t *testing.T) { //nolint:funlen // table-driven test
+	t.Parallel()
+
 	tests := []struct {
 		name       string
 		remoteAddr string
@@ -65,11 +68,15 @@ func TestRealIP(t *testing.T) {
 
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
-			req, _ := http.NewRequest(http.MethodGet, "/", nil)
+			t.Parallel()
+
+			req, _ := http.NewRequestWithContext(context.Background(), http.MethodGet, "/", nil)
 			req.RemoteAddr = tt.remoteAddr
+
 			if tt.xRealIP != "" {
 				req.Header.Set("X-Real-IP", tt.xRealIP)
 			}
+
 			if tt.xff != "" {
 				req.Header.Set("X-Forwarded-For", tt.xff)
 			}