Fix #145: tree membership management (list / add / role / remove)

TreeMembership was enforced on every read/write but had no API or UI to manage
members — trees were effectively single-user, breaking full-CRUD (NN#8).

Backend (/trees/{id}/members): list (members only — the list exposes emails, so
non-members never see it, even on public trees); add an existing user by email
(owner only, 404 if no such account, 409 if already a member); PATCH role;
DELETE. A tree must always keep ≥1 owner (demote/remove of the sole owner → 409).
All changes audited.

Frontend: a Members page (owner gets add-by-email + per-member role select +
remove; others see a read-only list) and a sidebar entry.

Test covers the full lifecycle + every guard. Suite 77 passed.

Closes #145

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Signed-off-by: Justin Paul <justin@jpaul.me>

backend/app/api/v1/__init__.py

@@ -9,6 +9,7 @@ from app.api.v1 import (
     events,
     gedcom,
     media,
+    members,
     names,
     persons,
     public,
@@ -32,3 +33,4 @@ api_router.include_router(media.router)
 api_router.include_router(gedcom.router)
 api_router.include_router(cleanup.router)
 api_router.include_router(public.router)
+api_router.include_router(members.router)