Migrate to normalized FK schema (domain_host, domain_tld)

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>

util.go

@@ -61,6 +61,12 @@ func stripTLD(host string) string {
 	return host
 }
 
+// getDomainHost extracts the host part from a full domain (without TLD)
+// e.g., "npr.org" -> "npr", "bbc.co.uk" -> "bbc.co"
+func getDomainHost(domain string) string {
+	return stripTLD(domain)
+}
+
 // fullHost reconstructs the full hostname from host and tld
 // e.g., ("example", "com") -> "example.com"
 func fullHost(host, tld string) string {