Require HTTPS for OIDC and public URLs; stop logging token responses

Address review findings on a10b24c:

- F1: the OIDC issuer and the discovered authorization, token and JWKS
  endpoints must be https://; plain http:// is allowed only for the
  loopback hosts localhost, 127.0.0.1 and [::1]. Config rejects a remote
  http:// issuer at startup; auth.New rejects a discovered remote http://
  endpoint, naming it, before it is used.
- F2: a failed code exchange is reduced to a fixed category plus the HTTP
  status; the token endpoint's body and error_description are never logged
  or wrapped into the returned error.
- F3: MEALQUEST_PUBLIC_URL must be https:// unless its host is loopback,
  so the Secure login cookie is always sent back on the callback.

The rule lives once, in internal/secureurl.Check.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/secureurl/secureurl_test.go

@@ -0,0 +1,40 @@
+package secureurl
+
+import (
+	"net/url"
+	"testing"
+)
+
+func TestCheck(t *testing.T) {
+	t.Parallel()
+	tests := []struct {
+		raw string
+		ok  bool
+	}{
+		{"https://meals.example.com", true},
+		{"https://auth.example.com/application/o/mealquest/", true},
+		{"https://192.0.2.10:8443", true},
+		{"http://localhost:8080", true},
+		{"http://LOCALHOST:8080", true},
+		{"http://127.0.0.1:8080/path", true},
+		{"http://[::1]:8080", true},
+		{"http://[::1]", true},
+		{"http://meals.example.com", false},
+		{"http://192.0.2.10", false},
+		{"http://127.0.0.2", false},
+		{"http://localhost.example.com", false},
+		{"http://0.0.0.0:8080", false},
+		{"ftp://localhost", false},
+		{"https://", false},
+		{"meals.example.com", false},
+	}
+	for _, tc := range tests {
+		u, err := url.Parse(tc.raw)
+		if err != nil {
+			t.Fatal(err)
+		}
+		if err := Check(u); (err == nil) != tc.ok {
+			t.Errorf("Check(%q) error = %v, want ok=%v", tc.raw, err, tc.ok)
+		}
+	}
+}