recordRequestStart() now performs a cheap DB lookup (isCustomHost) before
building the InterceptedRequest. Hosts registered in inspector_custom_hosts
with enabled=1 receive source="custom-host" and agent=undefined, so they
appear correctly under the "Custom" profile filter instead of being
silently routed to "agent-bridge" entries.
Added isCustomHost() helper to inspectorCustomHosts.ts and a unit test
covering enabled custom-host, non-custom host, and disabled custom-host cases.
