fix: Plugin migration runner + unified_search + marketplace migrations

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

app/plugins/migration_runner.py

@@ -95,8 +95,12 @@ class MigrationRunner:
         created_tables = new_tables - existing_tables
 
         # Validate: all newly created tables must have tenant_id column
+        # Exception: tables marked as global in the SQL with `-- GLOBAL TABLE` comment
+        global_tables = self._extract_global_table_names(sql_content)
         tables_without_tenant = []
         for table_name in created_tables:
+            if table_name in global_tables:
+                continue  # Global tables are exempt from tenant_id requirement
             if not await self._table_has_column_via_session(db, table_name, "tenant_id"):
                 tables_without_tenant.append(table_name)
 
@@ -105,7 +109,8 @@ class MigrationRunner:
             await db.rollback()
             raise MigrationValidationError(
                 f"Plugin '{plugin_name}' migration '{migration_filename}' created tables without tenant_id column: "
-                f"{', '.join(tables_without_tenant)}. All plugin tables must have tenant_id for multi-tenant isolation."
+                f"{', '.join(tables_without_tenant)}. All plugin tables must have tenant_id for multi-tenant isolation. "
+                f"If a table is intentionally global, add `-- GLOBAL TABLE: table_name` comment to the migration SQL."
             )
 
         # Record the migration in plugin_migrations table
@@ -469,3 +474,14 @@ class MigrationRunner:
         # Match CREATE TABLE [IF NOT EXISTS] "table_name" or CREATE TABLE table_name
         pattern = r'CREATE\s+TABLE\s+(?:IF\s+NOT\s+EXISTS\s+)?["\']?(\w+)["\']?'
         return re.findall(pattern, sql, re.IGNORECASE)
+
+    def _extract_global_table_names(sql: str) -> set[str]:
+        """Extract table names marked as global in SQL comments.
+
+        Looks for `-- GLOBAL TABLE: table_name` comments in the SQL.
+        These tables are exempt from the tenant_id column requirement.
+        """
+        import re
+
+        pattern = r'--\s*GLOBAL\s+TABLE:\s*(\w+)'
+        return set(re.findall(pattern, sql, re.IGNORECASE))