feat(alerts): per-source-group dedup so two failing backups produce two alerts

Until now the open-alert key was (host_id, kind, resolved_at IS NULL).
A host with two source groups both failing collapsed onto one
backup_failed row — second failure bumped last_seen_at and
overwrote the message but never re-fan-out. Operators saw one
alert that appeared to flap, not two distinct broken things.

Schema changes (column-level ALTER, no rebuild):

- 0015 jobs.source_group_id (FK → source_groups, ON DELETE SET NULL,
  index). Populated for backup jobs in CreateJob.
- 0016 alerts.dedup_key (NOT NULL DEFAULT ''). The old alerts_open
  partial index gets dropped and replaced with a UNIQUE partial
  index on (host_id, kind, dedup_key) WHERE resolved_at IS NULL —
  the index is now the actual dedup primitive.

Plumbing:

- RaiseOrTouch / AutoResolve / Alert struct gain dedup_key.
- engine.JobFinishedEvent gains SourceGroupID; handleJobFinished
  passes it through for backup_failed only (forget/prune/check stay
  repo-scoped with key='').
- ws.handler reads SourceGroupID off the freshly-loaded job row.
- dispatchJobWithPayload gains a *string sourceGroupID arg; the
  per-group Run-now path and schedule.fire path pass &g.ID.

Test coverage: TestRaiseOrTouchDedupsPerSourceGroup proves two
distinct groups produce two distinct open alerts and that resolving
one does not auto-resolve the other.

Dev tool: cmd/_fake_alert gains -dedup-key flag.

internal/server/http/schedule_push.go

@@ -180,13 +180,15 @@ func (s *Server) dispatchBackupForGroupCore(ctx context.Context, conn *ws.Conn,
 	jobID := ulid.Make().String()
 	now := time.Now().UTC()
 	scheduleRef := scheduleID
+	groupRef := g.ID
 	if err := s.deps.Store.CreateJob(ctx, store.Job{
-		ID:          jobID,
-		HostID:      hostID,
-		Kind:        string(api.JobBackup),
-		ScheduledID: &scheduleRef,
-		ActorKind:   "schedule",
-		CreatedAt:   now,
+		ID:            jobID,
+		HostID:        hostID,
+		Kind:          string(api.JobBackup),
+		ScheduledID:   &scheduleRef,
+		SourceGroupID: &groupRef,
+		ActorKind:     "schedule",
+		CreatedAt:     now,
 	}); err != nil {
 		slog.Warn("schedule.fire: persist job", "host_id", hostID,
 			"schedule_id", scheduleID, "group", g.Name, "err", err)