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.

server/db/migrations/20260502000000_add_scope_columns_to_webhook.js

@@ -0,0 +1,22 @@
+/*!
+ * Copyright (c) 2024 PLANKA Software GmbH
+ * Licensed under the Fair Use License: https://github.com/plankanban/planka/blob/master/LICENSE.md
+ */
+
+exports.up = (knex) =>
+  knex.schema.alterTable('webhook', (table) => {
+    table.bigInteger('project_id');
+    table.bigInteger('user_id');
+
+    table.index('project_id');
+    table.index('user_id');
+  });
+
+exports.down = (knex) =>
+  knex.schema.alterTable('webhook', (table) => {
+    table.dropIndex('user_id');
+    table.dropIndex('project_id');
+
+    table.dropColumn('user_id');
+    table.dropColumn('project_id');
+  });