Implement Phase 2: Database migrations and chat schema

[DaveAlessi]

Add custom migration system and chat persistence tables:

Migration Infrastructure (SNY-45):

• Created migration runner with transaction support and rollback
• Migration tracking via schema_migrations table
• CLI tools: npm run migrate, npm run migrate:status
• Added allAsync() helper to dbHelpers for multi-row queries
• Made createApp() async to support migration runner
• Migrations are idempotent and work with :memory: databases

Schema Migrations:

• 001_baseline.js - Captures current users table schema
• 002_chat_tables.js - Adds conversations, conversation_members, messages tables
• Foreign keys with CASCADE DELETE for referential integrity
• Performance indexes on conversation_id, created_at, deleted_at, user_id
• Soft delete support via deleted_at column in messages

Chat Repositories (SNY-48):

• conversationRepository.js - 7 methods for conversation CRUD and membership
• messageRepository.js - 7 methods for message CRUD with soft delete filtering
• Follows existing userRepository pattern (async, db-first param, Promise-based)

Developer Tools (SNY-51):

• seed.js script creates test users (alice, bob, charlie)
• Generates sample conversations and messages for local testing
• npm run seed:reset for clean test data

Database Schema:
Tables: users, conversations, conversation_members, messages, schema_migrations Indexes: 3 performance indexes for query optimization Total files: 9 created, 5 modified

All 59 tests passing. Zero breaking changes to existing functionality.

[linear-code]

SNY-64 Phase 2: Data model + migrations