feat: add structured plugin logging system with admin log viewer

Introduces Peptide_News_Logger class with leveled logging (info, warning,
error, debug) to a custom DB table, auto-pruning at 2000 rows. Adds a
filterable, paginated log viewer to the admin settings page. Instruments
the LLM and Fetcher classes with structured log calls for fetch cycles,
AI processing, and error tracking. Version bumped to 2.1.0.

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

includes/class-peptide-news-activator.php

@@ -15,6 +15,11 @@ class Peptide_News_Activator {
         self::create_tables();
         self::set_default_options();
         self::schedule_cron();
+
+        // Create the log table (idempotent via dbDelta).
+        if ( class_exists( 'Peptide_News_Logger' ) ) {
+            Peptide_News_Logger::create_table();
+        }
     }
 
     /**