db: treat unknown pre-auth key as not found

An unknown or deleted key returned a bare error matching neither the
not-found nor pre-auth-key checks, so registration returned a server error
instead of 401. Wrap gorm.ErrRecordNotFound.

Updates #3312

hscontrol/db/preauth_keys.go

@@ -15,7 +15,10 @@ import (
 )
 
 var (
-	ErrPreAuthKeyNotFound          = errors.New("auth-key not found")
+	// ErrPreAuthKeyNotFound wraps gorm.ErrRecordNotFound so an unknown or
+	// deleted key is treated as a missing record by callers, which the
+	// registration handler maps to a 401 rather than a raw server error.
+	ErrPreAuthKeyNotFound          = fmt.Errorf("auth-key not found: %w", gorm.ErrRecordNotFound)
 	ErrPreAuthKeyExpired           = errors.New("auth-key expired")
 	ErrSingleUseAuthKeyHasBeenUsed = errors.New("auth-key has already been used")
 	ErrUserMismatch                = errors.New("user mismatch")

hscontrol/db/preauth_keys_test.go

@@ -487,3 +487,16 @@ func TestUsePreAuthKeyAtomicCAS(t *testing.T) {
 		"second UsePreAuthKey error must be a PAKError, got: %v", err)
 	assert.Equal(t, "authkey already used", pakErr.Error())
 }
+
+// TestGetPreAuthKeyUnknownMapsToRecordNotFound ensures an unknown (or deleted)
+// pre-auth key resolves to a record-not-found error, which the registration
+// handler maps to a 401 rather than a raw server error.
+func TestGetPreAuthKeyUnknownMapsToRecordNotFound(t *testing.T) {
+	db, err := newSQLiteTestDB()
+	require.NoError(t, err)
+
+	_, err = db.GetPreAuthKey("nonexistent-key")
+	require.Error(t, err)
+	require.ErrorIs(t, err, gorm.ErrRecordNotFound,
+		"unknown pre-auth key must map to record-not-found (handled as 401)")
+}