docs: add ADMIN_API_KEY to .env.example with generation instructions

Includes commands for generating secure keys using Python or openssl.

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

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

.env.example

@@ -1,6 +1,27 @@
-# Database (port 5433 to avoid conflict with existing Postgres)
+# LetsBe Orchestrator Environment Variables
+# Copy this file to .env and fill in the values
+
+# =============================================================================
+# ADMIN API KEY (Required for production)
+# =============================================================================
+# Used to authenticate admin endpoints like registration token management.
+#
+# Generate a secure key with:
+#   python -c "import secrets; print(secrets.token_hex(32))"
+#
+# Or using openssl:
+#   openssl rand -hex 32
+#
+ADMIN_API_KEY=change-me-in-production
+
+# =============================================================================
+# Database
+# =============================================================================
+# Port 5433 to avoid conflict with existing Postgres
 DATABASE_URL=postgresql+asyncpg://orchestrator:orchestrator@localhost:5433/orchestrator
 
-# Application
+# =============================================================================
+# Application Settings
+# =============================================================================
 DEBUG=true
 APP_NAME=LetsBe Orchestrator