feat: add VariableKey class and docs

Introduce a final `VariableKey` class implementing `VariableKeyInterface` and a detailed docblock explaining its purpose for application-specific variable names stored in the `variables` table (activation codes, reset tokens, online session tokens, etc.).
Encourages using these constants instead of raw strings, includes an example; class currently inherits interface constants and can be extended with project-specific keys.

Author: bkader

src/config/constants.php

@@ -48,6 +48,34 @@ final class SessionKey implements SessionKeyInterface
 
 // --------------------------------------------------------------------
 
+/**
+ * VariableKey
+ *
+ * Application-specific variable keys extending the core system's
+ * {@see VariableKeyInterface}. These constants map to the `name` column in the
+ * `variables` table and should be used for per-entity state such as activation
+ * codes, password reset codes, quick-login tokens, and online session tokens.
+ *
+ * Developers are encouraged to use this class instead of raw variable-name
+ * strings when reading from or writing to the `variables` table. Applications
+ * may add project-specific variable keys here without modifying the core
+ * interface.
+ *
+ * @example
+ * ```php
+ * $this->db->get_where('variables', ['name' => VariableKey::ONLINE_TOKEN]);
+ * ```
+ *
+ * @since 0.0.1
+ */
+final class VariableKey implements VariableKeyInterface
+{
+	// Currently inherits all constants from VariableKeyInterface.
+	// Add application-specific variable keys here if needed.
+}
+
+// --------------------------------------------------------------------
+
 /**
  * UserLevel
  *