M7(1b/3): move clickhouse_schema.sql out of seed's *.up.sql glob

The seed service globs migrations/*.up.sql and applies them via
Postgres conn.Exec. 007_clickhouse.up.sql is pure ClickHouse DDL
(CREATE DATABASE, ENGINE = MergeTree, LowCardinality(String),
MATERIALIZED VIEW, etc.) and Postgres cannot parse it. The
archiverd's Go-side ensureCHSchema is the runtime source of truth;
this file is the hand-runnable ops reference. Renaming to
clickhouse_schema.sql (no .up.sql suffix) keeps it as a doc while
removing it from the seed path.