Phase 2: Shared calendars backend core + QA fixes
Router: invite/accept/reject flow, membership CRUD, event locking
(timed + permanent), sync endpoint, local color override.
Services: permission hierarchy, atomic lock acquisition, disconnect cascade.
Events: shared calendar scoping, permission/lock enforcement, updated_by tracking.
Admin: sharing-stats endpoint. nginx: rate limits for invite + sync.
QA fixes: C-01 (read-only invite gate), C-02 (updated_by in this_and_future),
W-01 (pre-commit response build), W-02 (owned calendar short-circuit),
W-03 (sync calendar_ids cap), W-04 (N+1 owner name batch fetch).
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>