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-config.php

@@ -0,0 +1,44 @@
+<?php
+/**
+ * Configuration constants for Peptide Search AI.
+ * Centralizes all magic numbers and configuration values.
+ *
+ * @package PeptideSearchAI
+ */
+
+if ( ! defined( 'ABSPATH' ) ) {
+	exit;
+}
+
+class PSA_Config {
+
+	// Search query validation
+	const MIN_QUERY_LENGTH = 2;
+	const MAX_QUERY_LENGTH = 100;
+
+	// Rate limiting (requests per hour)
+	const REST_RATE_LIMIT = 20;
+	const AJAX_RATE_LIMIT = 10;
+
+	// Daily generation cap
+	const DAILY_GENERATION_CAP   = 50;
+
+	// Generation timing
+	const PENDING_TIMEOUT        = 300; // seconds (5 minutes)
+	const MAX_GENERATION_RETRIES = 3;
+
+	// AI token limits
+	const VALIDATION_MAX_TOKENS  = 300;
+	const GENERATION_MAX_TOKENS  = 4096;
+
+	// Cache TTL
+	const VALIDATION_CACHE_TTL   = null; // Set to DAY_IN_SECONDS at runtime
+	const PUBCHEM_CACHE_TTL      = null; // Set to 7 * DAY_IN_SECONDS at runtime
+
+	// API retry settings
+	const API_RETRY_MAX          = 3;
+	const API_RETRY_BASE_DELAY   = 5; // seconds
+
+	// Cost tracking and budget
+	const DEFAULT_MONTHLY_BUDGET = 0; // 0 = unlimited
+}