Feature: Run audits concurrently using concurrent_tasks setting

When audit_only=True, all audit tasks across all snapshots are flattened
into a single thread pool instead of following DAG ordering. Since audits
are read-only SELECT queries with no side effects, DAG dependencies are
irrelevant and all concurrent_tasks worker slots stay filled.

Per-model audit concurrency is also plumbed through SnapshotEvaluator.audit()
via a new audit_concurrent_tasks parameter (defaults to sequential). The
cross-model path hardcodes this to 1 to avoid nested thread pool
multiplication.

The SnapshotEvaluator parameter ddl_concurrent_tasks is renamed to
concurrent_tasks to reflect its broader scope.

Closes #5468

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: airhorns

sqlmesh/core/context.py

@@ -492,7 +492,7 @@ def snapshot_evaluator(self) -> SnapshotEvaluator:
                     gateway: adapter.with_settings(execute_log_level=logging.INFO)
                     for gateway, adapter in self.engine_adapters.items()
                 },
-                ddl_concurrent_tasks=self.concurrent_tasks,
+                concurrent_tasks=self.concurrent_tasks,
                 selected_gateway=self.selected_gateway,
             )
         return self._snapshot_evaluator