Stop 500ing when a service account acts on an incident

Every incident-mutation handler read userFromContext(ctx) and wrote the
result's .ID into acknowledged_by/incident_events.user_id without checking
the ok bool. For a team-scoped service-account caller this returned a
zero-value user id, which violated the users(id) FK and 500'd on
acknowledge, unacknowledge, resolve, snooze, unsnooze and create-note.
handleDeleteNote didn't crash but silently matched zero rows instead
(WHERE user_id = 0), so a service account could never delete its own note.

Add acknowledged_by_service_account_id (incidents) and service_account_id
(incident_events) as nullable FKs to service_accounts(id), parallel to and
mutually exclusive with the existing human columns (migration 015, with a
CHECK enforcing the exclusion). Route every one of the six handlers plus
delete-note through a new callerActorIDs() helper that branches on
Caller.AsHuman()/ServiceAccountID() instead of assuming a human, and thread
a serviceAccountID parameter through logEvent and the new
acknowledgeIncidentAs (acknowledgeIncident itself is untouched: its only
other caller, the push-notification Acknowledge button, is always human).
Render the new actor distinctly from both a human and "the server acted"
in the web UI's incident timeline and facts card.

handleIncidentAssign, handleIncidentArchive and handleIncidentUnarchive are
deliberately not touched here — they track no actor at all today, for
anyone, which is a separate pre-existing gap (follow-up issue to come).

Fixes #25

internal/api/archiver.go

@@ -142,7 +142,7 @@ func expireStale(ctx context.Context, db *sql.DB, staleAfter time.Duration, skip
 			continue
 		}
 		alertID := id
-		if err := logEvent(ctx, db, incidentID, evAlertResolved, nil, &alertID, nil); err != nil {
+		if err := logEvent(ctx, db, incidentID, evAlertResolved, nil, nil, &alertID, nil); err != nil {
 			log.Printf("sweeper: log expiry event: %v", err)
 		}
 	}