Constant GET_SCHEMA_HASH_SQL 

pub const GET_SCHEMA_HASH_SQL: &str = "
SELECT schema_hash, payload FROM server_schema
WHERE proxy = ?1 AND upstream_url = ?2 AND method = ?3;
";

Fetch the current schema_hash and payload for a given proxy+upstream+method. Used by the writer to detect changes before upserting. Parameters: ?1=proxy, ?2=upstream_url, ?3=method.