Server halves of #17/#18/#19: PATCH /users/me and change-password
(verifies the current password, logs out every other session),
GET/DELETE /users/me/sessions with current-session flag and protection
against revoking oneself; InstanceSettingsService as a typed, cached,
Zod-validated registry over instance_settings (schema-default fallback
for invalid stored values, audit-logged writes) consumed by the signup
flow; /admin/settings behind the new SiteAdminGuard with strict
unknown-key rejection. SessionsService moves to its own module to keep
Auth/Users acyclic. Three new e2e suites bring the api to 42 tests.
Part of #17, #18, #19
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
