refactor(health-report): put summary where the other writer puts it

check_history has two producers. sysmon-go writes `summary` at the top level
beside `status`; this module wrote it under `metrics`. So a reader had to know
which producer wrote a row before it could find out what the row said, and a
query written the obvious way found one and silently missed the other.

That is the T-36 failure repeating. There, per-domain queries returned rows from
August and looked like a system that had stopped reporting, because the data was
nested under a composite row nobody had mentioned. Nothing was missing; the
query was asking the wrong shape. verify.sh had already grown a coalesce over
both spellings, which is the tell: a compatibility shim that hides a schema
disagreement rather than resolving it.

D-33 made this table a contract between producers, and a contract needs one
spelling.

Summary is now a required parameter with no default. sysmon-go enforces the same
thing through Domain.Run's signature, and the reason is identical: a row whose
substance is missing looks exactly like a row whose check found nothing to say.
Both call sites pass it; the failure path passes the exception rather than
leaving the field to the metrics blob.

Old rows keep the nested spelling and verify.sh keeps reading both, because
rewriting history to match a new convention is a worse trade than a fallback
with a reason attached.

Also drops "Three consequences" from the module docstring, which by then listed
five. A hardcoded count beside the thing it counts is the same defect as
install.sh printing "wrote 8 keys" while writing ten — this morning's bug, in
prose instead of code.

Co-Authored-By: Claude <noreply@anthropic.com>

src/executors/portainer_backup_executor.py

@@ -158,6 +158,7 @@ async def execute(config: dict, settings: Settings) -> str:
             domain="backup",
             status=health_report.CRITICAL,
             source="scheduler/portainer_backup_executor",
+            summary=f"backup failed: {str(exc)[:300]}",
             metrics={"job": "scheduler/portainer_backup_executor", "error": str(exc)[:400]},
         )
         raise
@@ -166,6 +167,7 @@ async def execute(config: dict, settings: Settings) -> str:
         domain="backup",
         status=health_report.OK,
         source="scheduler/portainer_backup_executor",
-        metrics={"job": "scheduler/portainer_backup_executor", "summary": output[:400]},
+        summary=output[:400],
+        metrics={"job": "scheduler/portainer_backup_executor"},
     )
     return output