Replace source_host column with proper FK to domains table using
composite key (domain_host, domain_tld). This enables JOIN queries
instead of string concatenation for domain lookups.
Changes:
- Update Feed struct: SourceHost/TLD → DomainHost/DomainTLD
- Update all SQL queries to use domain_host/domain_tld columns
- Add column aliases (as source_host) for API backwards compatibility
- Update trigram index from source_host to domain_host
- Add getDomainHost() helper for extracting host from domain
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
