feat: soporte Google Gemini como proveedor de IA

- Settings: agrega Gemini al selector de proveedor, campos gemini_api_key y gemini_model
- Modelos: gemini-2.0-flash, gemini-1.5-flash, gemini-1.5-pro
- AiBot::callGemini(): llama generateContent API con system_instruction y historial
- migrate.php: seed defaults gemini_api_key y gemini_model

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

services/AiBot.php

@@ -36,6 +36,7 @@ class AiBot
 
         return match ($provider) {
             'openai' => self::callOpenAI($systemPrompt, $history, $company),
+            'gemini' => self::callGemini($systemPrompt, $history),
             'mock'   => self::mockResponse($history),
             default  => self::callOpenAI($systemPrompt, $history, $company),
         };
@@ -104,6 +105,56 @@ class AiBot
         return ['content' => $text];
     }
 
+    private static function callGemini(string $systemPrompt, array $history): ?array
+    {
+        $apiKey = env('GEMINI_API_KEY', '');
+        if ($apiKey === '') return null;
+
+        $model = env('GEMINI_MODEL', 'gemini-2.0-flash');
+        $maxTokens = (int)env('AI_MAX_TOKENS', '500');
+
+        // Gemini usa "contents" con roles "user"/"model"
+        $contents = [];
+        foreach ($history as $msg) {
+            $role = ($msg['role'] ?? 'user') === 'assistant' ? 'model' : 'user';
+            $contents[] = ['role' => $role, 'parts' => [['text' => $msg['content'] ?? '']]];
+        }
+        // Si el historial está vacío o el último es del model, Gemini requiere que el último sea user
+        if (empty($contents)) {
+            $contents[] = ['role' => 'user', 'parts' => [['text' => '']]];
+        }
+
+        $payload = json_encode([
+            'system_instruction' => ['parts' => [['text' => $systemPrompt]]],
+            'contents'           => $contents,
+            'generationConfig'   => ['maxOutputTokens' => $maxTokens, 'temperature' => 0.7],
+        ]);
+
+        $url = "https://generativelanguage.googleapis.com/v1beta/models/{$model}:generateContent?key={$apiKey}";
+        $ch  = curl_init($url);
+        curl_setopt_array($ch, [
+            CURLOPT_POST           => true,
+            CURLOPT_POSTFIELDS     => $payload,
+            CURLOPT_HTTPHEADER     => ['Content-Type: application/json'],
+            CURLOPT_RETURNTRANSFER => true,
+            CURLOPT_TIMEOUT        => 30,
+        ]);
+
+        $response = curl_exec($ch);
+        $httpCode = curl_getinfo($ch, CURLINFO_HTTP_CODE);
+        $error    = curl_error($ch);
+        curl_close($ch);
+
+        if ($httpCode !== 200 || $response === false) {
+            return ['content' => 'Lo siento, tengo problemas para procesar tu mensaje. Por favor intenta de nuevo.'];
+        }
+
+        $data = json_decode($response, true);
+        $text = $data['candidates'][0]['content']['parts'][0]['text'] ?? null;
+
+        return $text !== null ? ['content' => $text] : ['content' => 'No pude generar una respuesta. ¿Puedes reformular tu pregunta?'];
+    }
+
     private static function mockResponse(array $history): array
     {
         $last = end($history);