Fix encryption key persistence for Docker environments

The check_encryption() function now properly handles Docker/container
environments where ROOTPATH/.env may be read-only or ephemeral.

Changes:
- Returns false when key persistence fails instead of always returning true
- Removes error suppression (@) to properly detect write failures
- Adds fallback to WRITEPATH/config/encryption.key for container volumes
- Splits logic into separate functions for clarity and testability

Fixes encryption key being lost on container restarts, which caused
stored passwords to become undecryptable.

GitHub-Issue: #4554

Add fallback key loading from WRITEPATH in Encryption config

When encryption key is not available from .env or environment variables,
the config now attempts to load from WRITEPATH/config/encryption.key.

This supports Docker environments where:
- .env file is read-only or ephemeral
- Key was persisted to the writable volume via check_encryption()

GitHub-Issue: #4554

Handle encryption unavailability gracefully in controllers

Changed EncrypterInterface property to nullable and added proper error
handling for cases where encryption key is not available.

Changes:
- Config controller: nullable encrypter property, try/catch around encryption
- Email_lib: check encryption before using encrypter
- Return meaningful error messages when encryption fails
- Log warnings when passwords saved without encryption

Users will now see clear error messages instead of unhandled exceptions
when encryption key cannot be initialized.

GitHub-Issue: #4554

Add encryption_failed error message to language file

Added localization string for encryption failure error messages.

GitHub-Issue: #4554

Add decrypt_value() and encrypt_value() helper functions

Extracts the recurring decryption/encryption pattern into reusable helper
functions with consistent error handling:

- decrypt_value(): Safely decrypts encrypted values with try/catch
- encrypt_value(): Safely encrypts values with error handling

Both functions handle:
- Empty/null values gracefully
- Missing encryption key (logs warning)
- Encryption/decryption failures (logs error, returns default)

This pattern appears in 8+ locations across the codebase.

GitHub-Issue: #4554

Refactor all encryption/decryption to use helper functions

Replaces direct encrypter calls with decrypt_value() and encrypt_value()
helpers throughout the codebase for consistent error handling:

- Config controller: SMTP, SMS, Mailchimp credential encryption
- Email_lib: SMTP password decryption
- Sms_lib: SMS password decryption
- Mailchimp_lib: API key decryption
- Customers controller: Mailchimp list ID decryption

Removes nullable EncrypterInterface property from Config controller as
encryption is now handled via helper functions.

GitHub-Issue: #4554

Address CodeRabbit feedback: validate key length, clarify encryption failure handling

- loadKeyFromWritable() now validates key length >= 64 before accepting
- encrypt_value() renamed  param, defaults to failing encryption required
- Clearer error message when credentials not saved

GitHub-Issue: #4554

fix: address CodeRabbit review comments for encryption key persistence

- Always mirror encryption key to both .env and WRITEPATH (Docker safety)
- Guard array key access with isset() before reading in Encryption.php
- Fix encrypt_value() to not treat string '0' as empty
- Improve error logging for failed encryption attempts

refactor: PSR-compliant naming and address objecttothis review comments

- Rename functions to camelCase: checkEncryption, writeEncryptionKeyToEnv, writeEncryptionKeyToWritable, loadEncryptionKeyFromWritable, abortEncryptionConversion, removeBackup, decryptValue, encryptValue
- Update all callers in Config.php, Customers.php, Migrations, Email_lib.php, Sms_lib.php, Mailchimp_lib.php
- Add EncryptionException import in security_helper.php (removed FQN)
- Use camelCase variables: $smtpPass, $emailConfig, $batchSaveData in affected files
- Remove unnecessary inline comments (code is self-documenting)
- Keep necessary docstrings for public API documentation

Address remaining CodeRabbit review comments

- Fix decryptValue() to use explicit null/empty check instead of empty()
  (handles string "0" correctly)
- Guard checkEncryption() result in migration before proceeding
- Check read success before writing backup restoration
- Consistent DIRECTORY_SEPARATOR usage in paths

GitHub-Issue: #4554

app/Config/Encryption.php

@@ -106,4 +106,54 @@ class Encryption extends BaseConfig
      * by CI3 Encryption default configuration.
      */
     public string $cipher = 'AES-256-CTR';
+
+    /**
+     * Constructor - loads encryption key from fallback location if not set.
+     *
+     * This supports Docker/container environments where ROOTPATH/.env may be
+     * read-only or ephemeral. The fallback key file is stored in WRITEPATH/config/.
+     */
+    public function __construct()
+    {
+        parent::__construct();
+
+        // If key not set from .env or environment, try WRITEPATH fallback
+        if (empty($this->key) || strlen($this->key) < 64) {
+            $fallbackKey = $this->loadKeyFromWritable();
+            if ($fallbackKey !== null) {
+                $this->key = $fallbackKey;
+            }
+        }
+    }
+
+    /**
+     * Loads encryption key from WRITEPATH/config/encryption.key.
+     *
+     * @return string|null The encryption key if found, null otherwise
+     */
+    private function loadKeyFromWritable(): ?string
+    {
+        $keyFile = WRITEPATH . 'config' . DIRECTORY_SEPARATOR . 'encryption.key';
+
+        if (!file_exists($keyFile) || !is_readable($keyFile)) {
+            return null;
+        }
+
+        $content = file_get_contents($keyFile);
+        if ($content === false) {
+            return null;
+        }
+
+        $data = json_decode($content, true);
+        if (
+            !is_array($data)
+            || !isset($data['key'])
+            || !is_string($data['key'])
+            || strlen($data['key']) < 64
+        ) {
+            return null;
+        }
+
+        return $data['key'];
+    }
 }