Covers Settings defaults, env var overrides, type validation,
edge cases, normalize_ollama_url, _compute_repo_root,
model_post_init, check_ollama_model_available,
get_effective_ollama_model, validate_startup, and APP_START_TIME.
Fixes#648
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>