Repoint a service's continuous archive source

Deliberately, explicitly moves where a service's continuous, standing archiving process writes: Postgres/Timescale's WAL-G archive_command, or MariaDB's binlog shipper. Both need everything written under one S3 prefix to stay restorable — data archived before this call lives under the previous source and will no longer be verifiable or replayable once archiving points at the new one.

This exists because a backup schedule that requests a different S3 source than the one archiving is currently pinned to is refused, not silently honoured (see ExternalServiceManager::repoint_continuous_archive_source for the incident this prevents). Call this endpoint to deliberately move the pin instead — for example, to switch a service from an operator's own S3 source onto Temps Cloud's managed one.