Add POST /webhook/{groupId} with per-group secrets (KV tenant:{groupId}), a custom provider (X-WebHooker-Signature HMAC, arbitrary JSON -> custom events through the route pipeline), and GitHub App installation isolation (Group.installationId) with automatic provisioning on installation.created (inst-{id} groups or binding matching owners groups). Includes WebhookPanel admin UI, custom route template, docs and 157 passing tests.
