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/ws/handler.go

@@ -219,12 +219,17 @@ func dispatchAgentMessage(ctx context.Context, c *Conn, hostID string, env api.E
 		}
 		if deps.AlertEngine != nil {
 			if job, err := deps.Store.GetJob(ctx, p.JobID); err == nil && job != nil {
+				groupID := ""
+				if job.SourceGroupID != nil {
+					groupID = *job.SourceGroupID
+				}
 				deps.AlertEngine.NotifyJobFinished(alert.JobFinishedEvent{
-					HostID: hostID,
-					JobID:  p.JobID,
-					Kind:   job.Kind,
-					Status: string(p.Status),
-					When:   p.FinishedAt,
+					HostID:        hostID,
+					JobID:         p.JobID,
+					Kind:          job.Kind,
+					Status:        string(p.Status),
+					SourceGroupID: groupID,
+					When:          p.FinishedAt,
 				})
 			}
 		}