Agent: new runner.BackupHooks struct + runHook helper invoked via
/bin/sh -c (cmd.exe /C on Windows). pre_hook non-zero exit aborts
the backup; post_hook always runs with RM_JOB_STATUS=succeeded|failed
in env. Output streamed as 'hook(<phase>): …' log.stream lines.
Hooks only run for kind=backup (other kinds skip both phases).
Server: resolveBackupHooks resolves group → host default → empty,
decrypts via crypto.AEAD with per-slot ad bytes, plumbs plaintext
into CommandRunPayload for both schedule.fire and per-group
Run-now dispatch sites. Decrypt failures degrade silently to no
hook so a malformed blob can't poison every backup.
Adds pre_hook/post_hook BLOB columns to source_groups and
pre_hook_default/post_hook_default to hosts. Bytes stored verbatim
(AEAD encrypt/decrypt happens at the HTTP layer where the AEAD key
lives). Round-trip tests cover set/clear semantics on both tables.
steve