feat: AI usage logging (AiLogger, ai_logs table, admin view)

- AiLogger.php: new service, inserts into ai_logs on every AI call
- ai_logs table: added to migrate.php (provider, model, call_type, tokens, duration_ms)
- AiBot: logs chat, media_response, nlu calls with timing and token counts
- MediaTranscriber: logs whisper, geminiAudio, openaiVision, geminiVision, claudeVision
- /admin/ai-logs: paginated table with provider/company filter
- Layout.php: added IA Logs nav link
- index.php: require AiLogger before AiBot; added GET /admin/ai-logs route

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

admin/Layout.php

@@ -53,6 +53,7 @@ class Layout
                 ['bot-config', '/admin/bot-config',        'bot-config', 'Bot'],
                 ['conv-flow',  '/admin/conversation-flow', 'conv-flow',  'Flujos'],
                 ['settings',   '/admin/settings',          'settings',   'Ajustes'],
+                ['ai-logs',    '/admin/ai-logs',            'live',       'IA Logs'],
             ],
         ];