feat(clickhouse): wire ClickHouseExecutionStore as active ExecutionStore

Add cameleer.storage.executions feature flag (default: clickhouse).
PostgresExecutionStore activates only when explicitly set to postgres.
Add by-seq snapshot endpoint for iteration-aware processor lookup.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

deploy/base/server.yaml

@@ -101,6 +101,8 @@ spec:
               value: "clickhouse"
             - name: CAMELEER_STORAGE_LOGS
               value: "clickhouse"
+            - name: CAMELEER_STORAGE_EXECUTIONS
+              value: "clickhouse"
 
           resources:
             requests: