P0-fix: plugin migrations use migration engine (crm_migration) instead of API engine (crm_api)

- main.py: registry.initialize(get_migration_engine()) instead of get_engine()
- main.py: plugin migrations run via get_migration_session_factory() not async_session()
- registry.py: upgrade_plugin, install_plugin, uninstall_plugin all use migration session for DDL
- db/__init__.py: get_migration_engine() raises RuntimeError if MIGRATION_DATABASE_URL missing (no fallback)
- Fixes fresh-install failure: crm_api has no DDL rights, plugin migrations need crm_migration

app/core/db/__init__.py

@@ -160,13 +160,23 @@ def get_worker_session_factory() -> async_sessionmaker[AsyncSession]:
 def get_migration_engine() -> AsyncEngine:
     """Get or create the migration engine (crm_migration role).
 
-    Used by Alembic for DDL operations. This engine connects as the table owner.
-    Falls back to the main engine if MIGRATION_DATABASE_URL is not set.
+    Used by Alembic and plugin migrations for DDL operations.
+    This engine connects as the table owner with BYPASSRLS.
+
+    Raises:
+        RuntimeError: If MIGRATION_DATABASE_URL is not set.
     """
     global _migration_engine
     if _migration_engine is None:
         settings = get_settings()
-        url = settings.migration_database_url or settings.database_url
+        url = settings.migration_database_url
+        if not url:
+            raise RuntimeError(
+                "MIGRATION_DATABASE_URL is not set. "
+                "Plugin migrations and Alembic require a dedicated migration "
+                "database connection (crm_migration role). "
+                "The application cannot start without it."
+            )
         _migration_engine = create_async_engine(
             url,
             pool_size=2,