hub v0.107.0: the hub rewrote a severity and said nothing (R-387); golden 0.223.0

One handler, two fields, opposite discipline. An unknown event_type is rejected
with a loud 400. An unknown severity was rewritten to "info" without a word -
and severityNotifies drops "info" before BOTH legs, so the event was stored,
answered 200, and mailed to nobody.

Two shipped features went out that way: DiskAlertKind.Severity emitted "warn"
until controller v0.215.0, app_start_failed until v0.223.0. Measured on the live
hub DB today: 91 app_start_failed events stored all-time, ZERO notification_log
rows before this session - not one, on any channel.

The mechanism built to catch this class was structurally blind to it: the
dispatcher's `unrecognized severity` line cannot execute for anything arriving
over the API, because the coercion one line earlier guarantees the value it
looks for cannot arrive.

The coercion STAYS - a rejected event is a lost event, and losing an alarm is
worse than mis-routing one. Only the silence is fixed: a WARN naming the
customer, the event type and the rejected value.

The dispatcher branch is KEPT, not deleted as dead, and the reason is evidence
rather than caution: cmd/hub/main.go wires dispatcher.ProcessEvent DIRECTLY as
the monitor.EventNotifyFunc for the staleness, host-staleness and offsite-box
checkers, which never pass through the handler. For those it is the only
severity guard there is. All 90 severity literals in internal/monitor are
already valid, so the guard is silent because the producers are correct.

Test count 702 -> 709. Red-proof seen failing: delete the WARN line and the
coercion test fails with "the hub rewrote a severity and said nothing".

Golden 0.223.0 baked and published (sha 9eaf39ac3921...), round-trip HTTP 206.
Vouching is the operator's act and was not done here.

manifests/hub.yaml

@@ -125,7 +125,7 @@ spec:
     spec:
       containers:
         - name: hub
-          image: gitea.dooplex.hu/admin/felhom-hub:0.106.0
+          image: gitea.dooplex.hu/admin/felhom-hub:0.107.0
           ports:
             - containerPort: 8080
               name: http