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.
