Add test coverage for FulfilContact and GmailService contact logic

[devinsays]

Context

After consolidating the three contact tables into fulfil_contacts (#51, #52), the only test coverage is in ActiveCustomersControllerTest which covers the API/controller layer. The core business logic has no tests.

Missing coverage

FulfilContact model

• Query scopes (broker(), discovered(), uncategorized(), fromFulfil())
• findOrCreateForContact() — idempotent create behavior
• recordEmailSent() / recordEmailReceived() — only updates when date is newer
• getFulfilName() / parseFulfilName() — broker name formatting

GmailService

• Email tracking updates during sync (outbound vs inbound direction)
• Contact discovery — creating discovered contacts for new email addresses matching a customer domain
• Broker email matching and tracking (prospect broker vs customer broker)
• buildBrokerEmailMap — aggregating broker emails across prospects and customers
• Deduplication — not creating contacts that already exist

ProspectController

• Prospect-to-customer conversion — migrating broker and uncategorized contacts to fulfil_contacts

LocalCustomerMetadata

• contacts() and brokerContacts() relationships
• hasBrokerData() using the consolidated relationship