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/ui_alerts_test.go

@@ -17,7 +17,7 @@ func TestAPIAlertsListsOpen(t *testing.T) {
 	srv, ts, st := rawTestServer(t)
 	hostID, _ := enrolHostForWS(t, srv, st, "host-alerts")
 	_, _, _ = st.RaiseOrTouch(context.Background(), hostID,
-		"backup_failed", "warning", "x", time.Now().UTC())
+		"backup_failed", "", "warning", "x", time.Now().UTC())
 	cookie := loginAsAdmin(t, st)
 
 	req, _ := stdhttp.NewRequest("GET", ts.URL+"/api/alerts?status=open", nil)