feat(diagnostics): crypto 健檢 + backup 目錄探測 + 15 個 usermeta 欄位（PR-G）

- Crypto::is_key_derivable()（A v3.3.2）：AUTH_KEY / SECURE_AUTH_SALT 皆缺
  時回 false，讓金鑰推導優雅短路
- wp tmdo doctor 新增兩項健檢：
  · backup 目錄 HTTP 可及性探測（200 = 紅旗並附 nginx 設定建議，
    403/404 = OK，0 = 離線時退回檢查 .htaccess）（A v3.1.9）
  · crypto 金鑰可推導性（缺常數時警告 notifier secrets 會以明文儲存）
- doctor_callback 呼叫簽章 3 參數 → 1 參數（A v3.0.3 修復）。**ABI 變更**：
  AddOn 註冊的 callback 若依賴 rows / full_name 需自行調整
- Member_Fields::register_admin_prefs_group() 由 7 欄擴到 22 欄（A v3.1.4），
  補上 wp_user_level / show_welcome_panel / wp_persisted_preferences /
  wp_user-settings / community-events-location 等 15 個 WP 原生 usermeta，
  這些先前全部落在 wp_usermeta
- capture_before_value 預設 true → false（A v3.1.5）：每次受管寫入省一次
  DB read。**行為變更**：需要 value_before 的消費者（audit log）要
  add_filter( 'wpdo_capture_before_value', '__return_true' ) 明確開啟

unit 451 / integration 398 GREEN

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TbG1keQQ7XBa7qMQY16KCY

includes/class-tmdo-crypto.php

@@ -23,7 +23,7 @@
  *   TMDO_Crypto::set_option( 'wpdo_slack_webhook', $url );   // writes v2
  *   $url = TMDO_Crypto::get_option( 'wpdo_slack_webhook' );  // reads v1 or v2
  *
- * @package WP_Data_Optimizer
+ * @package TMDO
  * @since   2.6.4 (v1 CBC)
  * @since   2.15.0 (v2 GCM, AEAD authentication)
  */
@@ -65,17 +65,34 @@ class TMDO_Crypto {
 	 * elsewhere. Both v1 and v2 use the same derived key (same secret material,
 	 * different cipher) so v1 ciphertext can be read after the v2 upgrade.
 	 *
-	 * Falls back to a sha256 of ABSPATH when constants are not defined
-	 * (unit-test environments). The fallback must be stable per request.
+	 * Returns empty string when both AUTH_KEY and SECURE_AUTH_SALT are absent —
+	 * callers treat '' as "encryption unavailable" and store plaintext instead.
 	 *
 	 * @return string 32 raw bytes.
 	 */
 	private static function derived_key(): string {
-		$salt  = defined( 'AUTH_KEY' ) ? AUTH_KEY : '';
-		$salt .= defined( 'SECURE_AUTH_SALT' ) ? SECURE_AUTH_SALT : ABSPATH;
+		$has_auth_key  = defined( 'AUTH_KEY' ) && AUTH_KEY !== '';
+		$has_auth_salt = defined( 'SECURE_AUTH_SALT' ) && SECURE_AUTH_SALT !== '';
+
+		if ( ! $has_auth_key && ! $has_auth_salt ) {
+			// Both salts missing — key would be derived from predictable ABSPATH. Refuse.
+			return '';
+		}
+
+		$salt  = $has_auth_key ? AUTH_KEY : '';
+		$salt .= $has_auth_salt ? SECURE_AUTH_SALT : '';
 		return substr( hash_hmac( 'sha256', 'wpdo_notifier_secrets_v1', $salt, true ), 0, 32 );
 	}
 
+	/**
+	 * Whether the site has valid WP auth constants for key derivation.
+	 *
+	 * @return bool False when both AUTH_KEY and SECURE_AUTH_SALT are absent/empty.
+	 */
+	public static function is_key_derivable(): bool {
+		return self::derived_key() !== '';
+	}
+
 	/**
 	 * Encrypt a plaintext string with AES-256-GCM (v2 format).
 	 *
@@ -89,13 +106,17 @@ class TMDO_Crypto {
 		if ( ! function_exists( 'openssl_encrypt' ) ) {
 			return $plaintext;
 		}
+		$key = self::derived_key();
+		if ( '' === $key ) {
+			return $plaintext;
+		}
 		$iv  = random_bytes( self::IV_LEN_V2 );
 		$tag = '';
 		// phpcs:ignore -- $tag is reference output for GCM auth tag.
 		$ciphertext = openssl_encrypt(
 			$plaintext,
 			self::CIPHER_V2,
-			self::derived_key(),
+			$key,
 			OPENSSL_RAW_DATA,
 			$iv,
 			$tag,
@@ -127,6 +148,9 @@ class TMDO_Crypto {
 		if ( ! function_exists( 'openssl_decrypt' ) ) {
 			return $stored;
 		}
+		if ( '' === self::derived_key() ) {
+			return $stored;
+		}
 		if ( str_starts_with( $stored, self::PREFIX_V2 ) ) {
 			return self::decrypt_v2( $stored );
 		}