Add local DB support for fetching dotrain source

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Summary

Currently, fetch_dotrain_source() always queries the metaboard subgraph (a remote GraphQL service) to resolve dotrain source code, even when the local DB is available and syncing on-chain events. This creates an unnecessary dependency on an external service and adds latency.

Goal

Enable the local DB sync pipeline to index MetaV1_2 events from the metaboard contract, so dotrain sources can be resolved locally without hitting the subgraph. This follows the same "local first, remote fallback" pattern already established for order fetching.

Implementation Steps

1. Add contract address to metaboard config — Add contract address field to MetaboardCfg #2444
2. Index metaboard events in the sync pipeline — Index metaboard contract events in the sync pipeline #2445
3. Use local DB in fetch_dotrain_source() — Use local DB for fetching dotrain source #2446