#91 FTX-90: Futex Audit & Hardening Fixes

Implement fixes proposed in futex_audit_report.md: 1. Fix test_web.py assertions (Completed) 2. Add explicit token format tests (Completed) 3. Remove legacy dict-return test seam in authn.py 4. Split main.py into routers (Tasks, Webhooks, Tenants) 5. Add updated_at trigger for Decision (Completed) 6. Enforce HITL_TOKEN_SECRET at startup (Completed) Note: Fix 3 and Fix 4 require extensive test suite updates due to mocked auth dicts and module movement. They are deferred to a dedicated refactoring PR.