Axum 0.7 uses :id path param syntax, not {id} (which is 0.8+).
The {id} routes silently returned 404 at runtime with no compile error.
Also adds comprehensive handler tests across all 6 remaining route
modules (ml, training, backtesting, performance, config, tune) covering
auth gating, no-service errors, and body validation — 37 new tests.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>