config: upgrade to 3-tier fallback system (Gemini → OpenCode → OpenRouter)

Added OpenRouter as tertiary fallback:
- Primary: Gemini (free tier, quota-limited)
- Secondary: OpenCode (fallback if Gemini quota exceeded)
- Tertiary: OpenRouter (final fallback, supports multiple models)

gemini-fallback.js now tries all three in sequence with proper error handling.

backend/src/utils/gemini-fallback.js

@@ -1,6 +1,6 @@
 /**
- * Gemini API with OpenCode Fallback
- * Tries Gemini first, falls back to OpenCode if quota exceeded
+ * Gemini API with Multi-Tier Fallback
+ * Tries: Gemini → OpenCode → OpenRouter
  */
 
 const fetch = require('node-fetch');
@@ -8,14 +8,16 @@ const fetch = require('node-fetch');
 const GEMINI_API_KEY = process.env.GOOGLE_API_KEY;
 const OPENCODE_API_KEY = process.env.OPENCODE_API_KEY;
 const OPENCODE_BASE_URL = process.env.OPENCODE_BASE_URL || 'https://api.opencode.com/v1';
+const OPENROUTER_API_KEY = process.env.OPENROUTER_API_KEY;
+const OPENROUTER_BASE_URL = process.env.OPENROUTER_BASE_URL || 'https://openrouter.ai/api/v1';
 
 async function generateWithFallback(prompt, options = {}) {
   console.log('🤖 Generating content...');
   
-  // Try Gemini first
+  // Tier 1: Try Gemini
   if (GEMINI_API_KEY) {
     try {
-      console.log('Attempting Gemini API...');
+      console.log('📍 Tier 1: Attempting Gemini API...');
       const response = await fetch(
         `https://generativelanguage.googleapis.com/v1/models/gemini-pro:generateContent?key=${GEMINI_API_KEY}`,
         {
@@ -35,20 +37,19 @@ async function generateWithFallback(prompt, options = {}) {
       }
       
       if (response.status === 429 || response.status === 403) {
-        console.warn('⚠️ Gemini quota exceeded, falling back to OpenCode...');
-        throw new Error('QUOTA_EXCEEDED');
+        console.warn('⚠️ Gemini quota exceeded, trying next fallback...');
+      } else {
+        throw new Error(`Gemini error: ${response.status}`);
       }
-      
-      throw new Error(`Gemini API error: ${response.status}`);
     } catch (err) {
       console.warn(`Gemini failed: ${err.message}`);
     }
   }
   
-  // Fallback to OpenCode
+  // Tier 2: Fallback to OpenCode
   if (OPENCODE_API_KEY) {
     try {
-      console.log('Attempting OpenCode API...');
+      console.log('📍 Tier 2: Attempting OpenCode API...');
       const response = await fetch(`${OPENCODE_BASE_URL}/chat/completions`, {
         method: 'POST',
         headers: {
@@ -69,19 +70,51 @@ async function generateWithFallback(prompt, options = {}) {
         return { success: true, provider: 'opencode', data };
       }
       
-      throw new Error(`OpenCode API error: ${response.status}`);
+      console.warn(`OpenCode error: ${response.status}, trying next fallback...`);
     } catch (err) {
-      console.error(`OpenCode failed: ${err.message}`);
+      console.warn(`OpenCode failed: ${err.message}`);
     }
   }
   
-  throw new Error('All generation APIs failed');
+  // Tier 3: Fallback to OpenRouter
+  if (OPENROUTER_API_KEY) {
+    try {
+      console.log('📍 Tier 3: Attempting OpenRouter API...');
+      const response = await fetch(`${OPENROUTER_BASE_URL}/chat/completions`, {
+        method: 'POST',
+        headers: {
+          'Authorization': `Bearer ${OPENROUTER_API_KEY}`,
+          'Content-Type': 'application/json',
+          'HTTP-Referer': 'https://gravl.app'
+        },
+        body: JSON.stringify({
+          model: options.model || 'openai/gpt-4',
+          messages: [{ role: 'user', content: prompt }],
+          temperature: options.temperature || 0.7,
+          max_tokens: options.maxTokens || 2048
+        })
+      });
+      
+      if (response.ok) {
+        const data = await response.json();
+        console.log('✅ OpenRouter API success');
+        return { success: true, provider: 'openrouter', data };
+      }
+      
+      throw new Error(`OpenRouter error: ${response.status}`);
+    } catch (err) {
+      console.error(`OpenRouter failed: ${err.message}`);
+    }
+  }
+  
+  throw new Error('All generation APIs failed (Gemini → OpenCode → OpenRouter)');
 }
 
 module.exports = {
   generateWithFallback,
   getAvailableProviders: () => ({
     gemini: !!GEMINI_API_KEY,
-    opencode: !!OPENCODE_API_KEY
+    opencode: !!OPENCODE_API_KEY,
+    openrouter: !!OPENROUTER_API_KEY
   })
 };