🛡️ Sentinel: L9 Commerce Engine Security Hardening and IDOR Fix

[dcplatforms]

This PR hardens the security posture of the L9 Commerce Engine by implementing zero-trust authentication checks and multi-tenant isolation.

Key changes:

1. JWT Failsafe: The authentication middleware now explicitly checks if JWT_SECRET is configured properly. If it is missing or set to the insecure development default, it returns a 500 error, preventing accidental insecure deployments.
2. IDOR Protection: The billing calculation endpoint now validates that the requested sessionId belongs to the authenticated user's fleet_id via a database join, preventing cross-tenant data access.
3. Bug Fixes: The invoice generation route was using undefined variables; these have been properly integrated with the InvoicingService.
4. Security Testing: A new test suite has been added to verify these security controls and prevent regressions.

Verified with 4/4 passing tests in services/09-commerce-engine/tests/security.test.js.

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PR created automatically by Jules for task 696326649137560831 started by @dcplatforms

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