feat: add cost/token tracking, monthly budget system, and dry-run mode

- Create PSA_Cost_Tracker class with database table for API call logging
- Log prompt/completion tokens and estimated cost for every OpenRouter API call
- Implement monthly budget system with hard stop (budget of 0 = unlimited)
- Add budget checks in validate_peptide_name() and generate_peptide_content()
- Add dry_run() method to estimate costs before making API calls
- Display monthly usage summary and recent API logs in admin settings
- Add monthly_budget setting to admin interface with number input field
- Update uninstall.php to drop psa_api_logs table on plugin removal

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

includes/class-psa-error.php

@@ -0,0 +1,39 @@
+<?php
+/**
+ * Standardized error handling for Peptide Search AI.
+ * Provides a unified interface for sending error responses with consistent structure.
+ *
+ * @package    PeptideSearchAI
+ * @deprecated 4.1.0 Currently unused. Retained for potential future integration.
+ */
+
+if ( ! defined( 'ABSPATH' ) ) {
+	exit;
+}
+
+class PSA_Error {
+
+	// Error type constants
+	const TYPE_VALIDATION = 'validation_error';
+	const TYPE_RATE_LIMIT = 'rate_limit_error';
+	const TYPE_SYSTEM = 'system_error';
+	const TYPE_NOT_FOUND = 'not_found';
+
+	/**
+	 * Send a standardized error response.
+	 *
+	 * @param string $type     The error type (one of the TYPE_* constants).
+	 * @param string $message  User-friendly error message.
+	 * @param int    $http_code HTTP response code (default 400).
+	 * @return void
+	 */
+	public static function send( $type, $message, $http_code = 400 ) {
+		wp_send_json_error(
+			array(
+				'error_type' => $type,
+				'message'    => $message,
+			),
+			$http_code
+		);
+	}
+}