feat: add webhook event history UI page (#164)

## Summary

Adds a per-app webhook event history page at `/apps/{id}/webhooks` showing received webhook events with match/no-match status.

## Changes

- **New template** `webhook_events.html` — displays webhook events in a table with time, event type, branch, commit SHA (linked when URL available), and match status badges
- **New handler** `HandleAppWebhookEvents()` in `webhook_events.go` — fetches app and its webhook events (limit 100)
- **New route** `GET /apps/{id}/webhooks` — registered in protected routes group
- **Template registration** — added `webhook_events.html` to the template cache in `templates.go`
- **Model enhancement** — added `ShortCommit()` method to `WebhookEvent` for truncated SHA display
- **App detail link** — added "Event History" link in the Webhook URL card on the app detail page

## UI

Follows the existing UI patterns (Tailwind CSS classes, Alpine.js `relativeTime`, badge styles, empty state, back-navigation). The page mirrors the deployments history page layout.

closes [#85](#85)

Co-authored-by: clawbot <clawbot@noreply.git.eeqj.de>
Reviewed-on: #164
Co-authored-by: clawbot <clawbot@noreply.example.org>
Co-committed-by: clawbot <clawbot@noreply.example.org>

internal/handlers/webhook_events.go

@@ -0,0 +1,56 @@
+package handlers
+
+import (
+	"net/http"
+
+	"github.com/go-chi/chi/v5"
+
+	"sneak.berlin/go/upaas/internal/models"
+	"sneak.berlin/go/upaas/templates"
+)
+
+// webhookEventsLimit is the number of webhook events to show in history.
+const webhookEventsLimit = 100
+
+// HandleAppWebhookEvents returns the webhook event history handler.
+func (h *Handlers) HandleAppWebhookEvents() http.HandlerFunc {
+	tmpl := templates.GetParsed()
+
+	return func(writer http.ResponseWriter, request *http.Request) {
+		appID := chi.URLParam(request, "id")
+
+		application, findErr := models.FindApp(request.Context(), h.db, appID)
+		if findErr != nil {
+			h.log.Error("failed to find app", "error", findErr)
+			http.Error(writer, "Internal Server Error", http.StatusInternalServerError)
+
+			return
+		}
+
+		if application == nil {
+			http.NotFound(writer, request)
+
+			return
+		}
+
+		events, eventsErr := application.GetWebhookEvents(
+			request.Context(),
+			webhookEventsLimit,
+		)
+		if eventsErr != nil {
+			h.log.Error("failed to get webhook events",
+				"error", eventsErr,
+				"app", appID,
+			)
+
+			events = []*models.WebhookEvent{}
+		}
+
+		data := h.addGlobals(map[string]any{
+			"App":    application,
+			"Events": events,
+		}, request)
+
+		h.renderTemplate(writer, tmpl, "webhook_events.html", data)
+	}
+}