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/handlers/port_validation_test.go

@@ -1,8 +1,10 @@
-package handlers
+package handlers //nolint:testpackage // tests unexported parsePortValues function
 
 import "testing"
 
 func TestParsePortValues(t *testing.T) {
+	t.Parallel()
+
 	tests := []struct {
 		name      string
 		host      string
@@ -24,6 +26,8 @@ func TestParsePortValues(t *testing.T) {
 
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
 			host, cont, valid := parsePortValues(tt.host, tt.container)
 			if host != tt.wantHost || cont != tt.wantCont || valid != tt.wantValid {
 				t.Errorf("parsePortValues(%q, %q) = (%d, %d, %v), want (%d, %d, %v)",