feat(auth): observability baseline + contract exercise

[mxllv]

Defines and wires a lightweight auth observability/audit baseline and adds contract-layer drift checks.

• Adds shared structured logger helper in @qyou/config and uses it across API + stellar-service auth modules (AUTH-112)
• Adds docs for auth observability/audit baseline (AUTH-111)
• Adds docs + audit script for shared auth contract layer (AUTH-110)
• Adds npm run auth:contracts and npm run auth:observability to exercise/monitor the baselines (AUTH-109)

Closes #427
Closes #428
Closes #429
Closes #430