docs: improve .env.example with GitHub OAuth setup guide

- Add clear section headers for organization
- Document GitHub OAuth setup for repo import feature
- Explain separation from Supabase login OAuth
- Add instructions for dev vs production setup

Author: DevanshuNEU

backend/.env.example

@@ -1,39 +1,88 @@
-# API Keys
+# =============================================================================
+# OpenCodeIntel Backend Environment Variables
+# =============================================================================
+
+# -----------------------------------------------------------------------------
+# AI & Search APIs
+# -----------------------------------------------------------------------------
+
+# OpenAI - for embeddings and AI features
 OPENAI_API_KEY=your_openai_api_key_here
+
+# Pinecone - vector database for semantic search
 PINECONE_API_KEY=your_pinecone_api_key_here
 PINECONE_INDEX_NAME=codeintel
 
-# Search V2 - Cohere Reranking (optional but recommended)
+# Cohere - reranking for search quality (optional but recommended)
 COHERE_API_KEY=your_cohere_api_key_here
 
-# Supabase
+# Voyage AI - code-specific embeddings (recommended for code search)
+# Get API key from https://dash.voyageai.com/
+VOYAGE_API_KEY=your_voyage_api_key_here
+
+# -----------------------------------------------------------------------------
+# Supabase - Authentication & Database
+# -----------------------------------------------------------------------------
+
 SUPABASE_URL=https://your-project.supabase.co
 SUPABASE_ANON_KEY=your_supabase_anon_key_here
 SUPABASE_JWT_SECRET=your_jwt_secret_here
+
+# Service role key - required for server-side database access (e.g., storing GitHub tokens)
+# Get from Supabase Dashboard → Settings → API → service_role key
 SUPABASE_SERVICE_ROLE_KEY=your_service_role_key_here
 
-# GitHub OAuth (for repo import feature)
+# -----------------------------------------------------------------------------
+# GitHub OAuth - One-Click Repo Import Feature
+# -----------------------------------------------------------------------------
+# IMPORTANT: This is SEPARATE from Supabase GitHub login!
+# 
+# Supabase login uses its own OAuth app (configured in Supabase Dashboard).
+# This OAuth app is ONLY for importing repositories from GitHub.
+#
+# Setup:
+# 1. Go to GitHub → Settings → Developer settings → OAuth Apps → New OAuth App
+# 2. Create app with name like "YourApp Repo Import"
+# 3. Set callback URL based on environment:
+#    - Development: http://localhost:3000/github/callback
+#    - Production:  https://yourdomain.com/github/callback
+# 4. Copy Client ID and generate Client Secret
+#
+# You may need separate OAuth apps for dev and production (different callback URLs)
+
 GITHUB_CLIENT_ID=your_github_oauth_app_client_id
 GITHUB_CLIENT_SECRET=your_github_oauth_app_client_secret
+
+# Must match EXACTLY what you set in GitHub OAuth App settings
 GITHUB_REDIRECT_URI=http://localhost:3000/github/callback
+
+# Frontend URL for redirects after OAuth
 FRONTEND_URL=http://localhost:3000
 
-# Backend API
+# -----------------------------------------------------------------------------
+# Backend Configuration
+# -----------------------------------------------------------------------------
+
 BACKEND_API_URL=http://localhost:8000
 API_KEY=dev-secret-key
 
-# CORS Configuration (Security)
+# CORS - allowed frontend origins (comma-separated for multiple)
 ALLOWED_ORIGINS=http://localhost:3000
 
-# Redis Cache
+# -----------------------------------------------------------------------------
+# Redis Cache (Optional)
+# -----------------------------------------------------------------------------
+
 REDIS_HOST=localhost
 REDIS_PORT=6379
 
-# Sentry Error Tracking (Optional)
+# -----------------------------------------------------------------------------
+# Monitoring & Debugging
+# -----------------------------------------------------------------------------
+
+# Sentry error tracking (optional)
 # Get DSN from https://sentry.io → Settings → Projects → Client Keys
 SENTRY_DSN=
-ENVIRONMENT=development
 
-# Search V3 - Voyage AI Code Embeddings (recommended for code search)
-# Get API key from https://dash.voyageai.com/
-VOYAGE_API_KEY=your_voyage_api_key_here
+# Environment identifier
+ENVIRONMENT=development