feat: Update .env.example to remove Gemini API configuration and clarify Vertex AI settings

Author: smirk-dev

backend/.env.example

@@ -1,18 +1,11 @@
 # =============================================================================
 # LegalMind Configuration
-# Google Cloud / Gemini API Settings
+# Google Cloud / Vertex AI Settings
 # =============================================================================
 
 # -----------------------------------------------------------------------------
-# Gemini API Configuration
+# Vertex AI Configuration
 # -----------------------------------------------------------------------------
-GEMINI_API_KEY=your-gemini-api-key-here
-
-# Use Vertex AI instead of public Gemini API (recommended for production)
-# Set to "true" to use Vertex AI with service account credentials
-# Set to "false" to use public Gemini API with GEMINI_API_KEY
-USE_VERTEX_AI=true
-
 # Model configuration (Gemini 2.0 Flash recommended for hackathon)
 GEMINI_MODEL=gemini-2.0-flash
 # Alternative: gemini-1.5-pro for more complex reasoning