Add support for defining ranking profiles in schema YAML, allowing
deployments to override builtin profiles with deployment-specific
signal names and tuning parameters.
- Add override_register() to ProfileRegistry for clean builtin replacement
- Add with_profiles() to TidalDbBuilder to thread schema profiles
- Parse profiles section in config.rs with full sort/strategy/agg support
- Validate that profile signal references exist in schema at startup
- Change load_schema() to return (Schema, Vec<RankingProfile>)
This closes the gap where the builtin for_you profile referenced
signals (view, like, share) that don't exist in deployment schemas,
causing all feed scores to normalize to 1.0.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>