Use local DB for fetching dotrain source

[findolor]

Summary

fetch_dotrain_source() currently always queries the metaboard subgraph to resolve dotrain source code from a dotrain_hash. With metaboard events now indexed locally (see #2445), the function should check the local DB first and only fall back to the subgraph when the data isn't available locally.

Problem

Even when the local DB is available and fully synced, every call to fetch_dotrain_source() makes a network request to the metaboard subgraph. This adds latency and creates an unnecessary dependency on an external service when the data is already present in the local meta_events table.

Proposed Change

Update fetch_dotrain_source() to follow the same "local first, remote fallback" pattern already used by get_order_by_hash():

1. Query the local DB meta_events table by (chain_id, subject) where subject is the dotrain_hash
2. If found, CBOR-decode the meta bytes and extract the DotrainSourceV1
3. If not found or local DB is unavailable, fall back to the existing metaboard subgraph query

Depends on

• Index metaboard contract events in the sync pipeline #2445