Fixes 3 issues found by API integration tests:
1. migration_runner.py: Add GLOBAL TABLE exemption for tables without tenant_id
- New _extract_global_table_names() method parses -- GLOBAL TABLE: comments
- marketplace_listings is intentionally global (no tenant_id)
2. unified_search/migrations/0002_embeddings.sql: Remove companies table (does not exist),
add DO $$ BEGIN END $$ blocks to check table existence before ALTER
3. marketplace/migrations/0001_initial.sql: Add -- GLOBAL TABLE: marketplace_listings comment