build: add axum 0.8 dependency; common: add Clock trait
Added axum 0.8 with minimal features (http1, json, tokio) to support
the in-process fake authorization server. Created Clock trait in
src/common/oauth/clock.rs with RealClock implementation returning
Unix seconds. Includes smoke test verifying reasonable timestamp.
Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>