Consolidate three fulfil contact tables into one

[devinsays]

Summary

Closes #51

• Merges fulfil_contact_metadata, fulfil_broker_contacts, and fulfil_uncategorized_contacts into a single fulfil_contacts table with type and source columns to preserve semantic distinctions
• Replaces three near-identical models with a single FulfilContact model using query scopes (broker(), discovered(), uncategorized(), etc.)
• Migration handles data transfer from all three tables and drops the old ones (reversible)

Motivation

All three tables had nearly identical schemas (same fulfil_party_id, email, last_emailed_at, last_received_at) and duplicate model logic (recordEmailSent, recordEmailReceived). GmailService had to query across all three to build a full contact picture. This consolidation simplifies queries, eliminates code duplication, and makes the contact model easier to extend.

Migration notes

• The migration copies data from all three old tables into fulfil_contacts, then drops them
• Uses INSERT IGNORE (MySQL) / INSERT OR IGNORE (SQLite) to handle any duplicate (fulfil_party_id, email) pairs across the old tables
• The down() migration reverses the process if needed

Test plan

• All 16 ActiveCustomersController tests pass
• Full test suite passes (157 pass, 3 pre-existing DomPDF failures)
• Verify migration runs cleanly on production (108 rows in fulfil_contact_metadata, plus broker and uncategorized contacts)
• Spot-check a customer's contact data after deploy to confirm no data loss