feat: filter webhooks per project, board, and account

Adds optional project, board, and acting-user scope to each webhook so
events from a specific board or project can be routed to a single
endpoint without an external automation tool in the middle (closes
#1457).

A webhook now fires for an event only when every set scope matches:
empty scope still means "fire for everything", keeping existing
webhooks working unchanged. Scope filtering is centralized in the
sendWebhooks helper, which auto-derives projectId/boardId from the
event payload, so existing call sites are untouched.

client/src/models/Webhook.js

@@ -18,11 +18,21 @@ export default class extends BaseModel {
     accessToken: attr(),
     events: attr(),
     excludedEvents: attr(),
+    projectId: fk({
+      to: 'Project',
+      as: 'project',
+      relatedName: 'webhooks',
+    }),
     boardId: fk({
       to: 'Board',
       as: 'board',
       relatedName: 'webhooks',
     }),
+    userId: fk({
+      to: 'User',
+      as: 'user',
+      relatedName: 'webhooks',
+    }),
   };
 
   static reducer({ type, payload }, Webhook) {