Add per-delivery replay to the event log (closes #203)

A delivery that exhausted max_retries was failed forever. The event
body is durably stored, so the only way to get it delivered was to
download it and re-POST by hand.

The event log now offers a Replay action on any finished delivery.
Replay creates a NEW pending delivery for the same event and target
and hands it to the delivery engine through the same Notifier the
receiver uses, so it is retried, SSRF-guarded and circuit-broken
exactly as a first attempt. The original delivery's status,
timestamps and recorded attempts are never touched, and what is
re-sent is the stored event body, not the response the original
attempt received.

The target is read as it stands now, including soft-deleted rows so
that a deleted target refuses the replay with a message on the page
instead of erroring or delivering from stale configuration. A
deactivated target and a target id that names nothing refuse the same
way, as does a replay of a delivery the engine has not finished.

Two bounds on replay storms: the route carries a per-client POST rate
limit of 30 per minute, and the handler refuses a replay while an
earlier one for the same event and target is still pending or
retrying.

One new metric, webhooker_delivery_replays_total, on the existing
target_type label. A replay is a real delivery and moves the attempt,
outcome and duration series like any other; this counter is what
separates it from ordinary traffic without adding a dimension to
every existing series.

The delivery row is written with associations omitted and with
neither Event nor Target populated, so no target row reaches the
per-webhook event database.

internal/handlers/source_management.go

@@ -781,16 +781,25 @@ func (h *Handlers) HandleSourceLogs() http.HandlerFunc {
 			totalPages++
 		}
 
+		// The banner a replay POST redirected back with. The
+		// message comes from a fixed set keyed by the outcome
+		// code, never from the query string itself.
+		replayMsg, replayOK := replayOutcome(
+			r.URL.Query().Get(replayOutcomeParam),
+		)
+
 		data := map[string]any{
-			tmplKeyWebhook: &webhook,
-			"Events":       evts,
-			"Page":         page,
-			"TotalPages":   totalPages,
-			"TotalEvents":  total,
-			"HasPrev":      page > 1,
-			"HasNext":      page < totalPages,
-			"PrevPage":     page - 1,
-			"NextPage":     page + 1,
+			tmplKeyWebhook:  &webhook,
+			"Events":        evts,
+			"ReplayMessage": replayMsg,
+			"ReplayQueued":  replayOK,
+			"Page":          page,
+			"TotalPages":    totalPages,
+			"TotalEvents":   total,
+			"HasPrev":       page > 1,
+			"HasNext":       page < totalPages,
+			"PrevPage":      page - 1,
+			"NextPage":      page + 1,
 		}
 
 		h.renderTemplate(w, r, "source_logs.html", data)