Resubmit a stored event as a new undelivered event (closes #250)

Capturing real webhook traffic and firing it repeatedly at a backend
under development is a primary function of this service, and
per-delivery replay cannot do it: it only ever resolves the delivery's
own original target, so a target created for a dev backend has no
prior delivery and nothing can be replayed to it.

The event log now offers a per-event Resubmit action. It stores a NEW
event copying the stored one's method, headers, body and content type
verbatim, and fans it out to the webhook's currently ACTIVE targets,
resolved fresh by the query the receiver uses -- so a target created
long after the original event arrived receives it. The original
event's deliveries have no bearing on where the copy goes, inactive
targets are skipped as the receiver skips them, and the action is
repeatable: replay's in-flight refusal is deliberately not ported,
because firing one captured event over and over is the point.

The receiver and the resubmit path share one construction and one
fan-out site. An eventSource value carries where the fields came from,
live request or stored event, and createAndFanOut writes the event and
its pending deliveries in one transaction and hands the tasks to the
same Notifier, so a resubmitted delivery is retried, SSRF-guarded and
circuit-broken exactly as a first one is. buildDeliveryTasks returns
an error instead of writing a response, which is what lets both
callers share it.

The stored event is read once, before the write transaction, with a
cast to blob, so a body over delivery.MaxInlineBodySize is copied byte
for byte and the engine loads it from the new event row.

A nullable resubmitted_from_id records provenance -- empty for an
event that arrived on the receiver -- and the event log reports the
relationship in both directions, without which the log is unreadable
after a few resubmits of one event. The route sits in the owned-source
group, so auth, CSRF and the body cap apply, with its own rate limit
bucket and an events_resubmitted_total counter.

Inbound signature verification is not re-run: there is no inbound
signature to check on a copy an authenticated, CSRF-protected operator
action submits.

Per-delivery replay is unchanged; it serves recovery, which resubmit
does not replace. The README claimed in four places that replay was
unimplemented, one of them telling the operator that a delivery
stranded by a target type change was lost; all four are corrected and
resubmit is documented beside replay.

internal/database/model_event.go

@@ -13,6 +13,14 @@ type Event struct {
 	Body        string `gorm:"type:text"   json:"body"`
 	ContentType string `json:"contentType"`
 
+	// ResubmittedFromID names the event this one was copied from by
+	// an operator resubmit. It is nil for an event that arrived on
+	// the receiver, which is every event created before the column
+	// existed. It is not a foreign key: the source event can be
+	// reaped by retention while its copies remain, and the id is
+	// kept as the record of where the copy came from either way.
+	ResubmittedFromID *string `gorm:"type:uuid;index" json:"resubmittedFromId,omitempty"`
+
 	// Relations
 	Webhook    Webhook    `json:"webhook,omitzero"`
 	Entrypoint Entrypoint `json:"entrypoint,omitzero"`