fix(review): harden payment, oauth, and migration paths

backend/internal/payment/wire.go

@@ -4,6 +4,7 @@ import (
 	"encoding/hex"
 	"fmt"
 	"log/slog"
+	"strings"
 
 	dbent "github.com/Wei-Shaw/sub2api/ent"
 	"github.com/Wei-Shaw/sub2api/internal/config"
@@ -19,11 +20,22 @@ type EncryptionKey []byte
 // When the key is non-empty but invalid (bad hex or wrong length), an error is returned
 // to prevent startup with a misconfigured encryption key.
 func ProvideEncryptionKey(cfg *config.Config) (EncryptionKey, error) {
-	if cfg.Totp.EncryptionKey == "" {
+	if cfg == nil {
+		slog.Warn("payment encryption key not configured — encrypted payment config and resume signing will be unavailable")
+		return nil, nil
+	}
+	keyHex := strings.TrimSpace(cfg.Totp.EncryptionKey)
+	if keyHex == "" {
 		slog.Warn("payment encryption key not configured — encrypted payment config will be unavailable")
 		return nil, nil
 	}
-	key, err := hex.DecodeString(cfg.Totp.EncryptionKey)
+	// Reject auto-generated TOTP keys for payment signing.
+	// They change across restarts/instances and can silently break resume-token flows.
+	if !cfg.Totp.EncryptionKeyConfigured {
+		slog.Warn("payment encryption/signing key is not explicitly configured; set TOTP_ENCRYPTION_KEY to enable payment resume tokens")
+		return nil, nil
+	}
+	key, err := hex.DecodeString(keyHex)
 	if err != nil {
 		return nil, fmt.Errorf("invalid payment encryption key (hex decode): %w", err)
 	}

backend/internal/payment/wire_test.go

@@ -0,0 +1,62 @@
+package payment
+
+import (
+	"strings"
+	"testing"
+
+	"github.com/Wei-Shaw/sub2api/internal/config"
+)
+
+func TestProvideEncryptionKeySkipsAutoGeneratedTotpKey(t *testing.T) {
+	t.Parallel()
+
+	cfg := &config.Config{
+		Totp: config.TotpConfig{
+			EncryptionKey:           strings.Repeat("a", 64),
+			EncryptionKeyConfigured: false,
+		},
+	}
+
+	key, err := ProvideEncryptionKey(cfg)
+	if err != nil {
+		t.Fatalf("ProvideEncryptionKey returned error: %v", err)
+	}
+	if len(key) != 0 {
+		t.Fatalf("encryption key len = %d, want 0", len(key))
+	}
+}
+
+func TestProvideEncryptionKeyUsesConfiguredTotpKey(t *testing.T) {
+	t.Parallel()
+
+	cfg := &config.Config{
+		Totp: config.TotpConfig{
+			EncryptionKey:           "0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef",
+			EncryptionKeyConfigured: true,
+		},
+	}
+
+	key, err := ProvideEncryptionKey(cfg)
+	if err != nil {
+		t.Fatalf("ProvideEncryptionKey returned error: %v", err)
+	}
+	if len(key) != 32 {
+		t.Fatalf("encryption key len = %d, want 32", len(key))
+	}
+}
+
+func TestProvideEncryptionKeyRejectsConfiguredInvalidLength(t *testing.T) {
+	t.Parallel()
+
+	cfg := &config.Config{
+		Totp: config.TotpConfig{
+			EncryptionKey:           "abcd",
+			EncryptionKeyConfigured: true,
+		},
+	}
+
+	_, err := ProvideEncryptionKey(cfg)
+	if err == nil {
+		t.Fatal("expected error for invalid key length")
+	}
+}