fix(github): address review — null actor, role gating, configured guard, scoped uninstall broadcast

- listeners: use optionalUUID(e.ActorID) so the system actor on the github-driven issue:updated event no longer panics activity / notification listeners; merged-PR → issue done now produces a status_changed activity and inbox entry.
- IntegrationsTab: gate the admin-only installations query on canManage so members no longer hit /github/installations 403; the configured/not-configured copy is also scoped to admins.
- backend: introduce isGitHubConfigured() requiring both GITHUB_APP_SLUG and GITHUB_WEBHOOK_SECRET, and surface that single flag from list-installations + connect endpoints so the frontend Connect button stays disabled until both are set.
- DeleteGitHubInstallationByInstallationID now RETURNs workspace_id; webhook handler publishes github_installation:deleted scoped to the right workspace so already-open Settings tabs invalidate in real time. ErrNoRows on a re-fired delete short-circuits cleanly.
- tests: focused webhook integration coverage (auto-link + merge → done, cancelled preservation, uninstall returns workspace).

Co-authored-by: multica-agent <github@multica.ai>

server/pkg/db/queries/github.sql

@@ -33,8 +33,9 @@ RETURNING *;
 -- name: DeleteGitHubInstallation :exec
 DELETE FROM github_installation WHERE id = $1 AND workspace_id = $2;
 
--- name: DeleteGitHubInstallationByInstallationID :exec
-DELETE FROM github_installation WHERE installation_id = $1;
+-- name: DeleteGitHubInstallationByInstallationID :one
+DELETE FROM github_installation WHERE installation_id = $1
+RETURNING id, workspace_id;
 
 -- =====================
 -- GitHub Pull Request