Use environment variables for OAuth URLs

Remove hard-coded URLs from OAuth configuration and use environment
variables instead for better security and configurability:

- Add CLERK_ISSUER and BASE_URL environment variables
- Update asgi_app.py OAuth endpoints to use env vars
- Update oauth_router.py to use configurable URLs
- Update .env.example with new environment variables
- Fix fetch tool bug: doc.content → doc.markdown_content

Environment variables:
- CLERK_ISSUER: Clerk domain issuer URL
- BASE_URL: Base URL for OAuth callbacks and API URLs
- CLERK_DOMAIN: Clerk domain name

🤖 Generated with [Claude Code](https://claude.ai/code)

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

.env.example

@@ -22,6 +22,7 @@ CLERK_FRONTEND_URL=http://localhost:3000
 
 # Clerk domain issuer (usually auto-configured)
 CLERK_ISSUER=https://your-clerk-domain.clerk.accounts.dev
+CLERK_DOMAIN=your-clerk-domain
 
 # =============================================================================
 # GOOGLE OAUTH SETTINGS
@@ -52,6 +53,9 @@ HOST=0.0.0.0
 PORT=8000
 LOG_LEVEL=info
 
+# Base URL for the application (used for OAuth callbacks and API URLs)
+BASE_URL=http://localhost:8000
+
 # =============================================================================
 # MCP SERVER SETTINGS
 # =============================================================================