feat(api): app members CRUD endpoints

Adds /api/v1/admin/apps/{id_or_slug}/members[/{user_id}]:

- GET    list members (joined with admin_users via list_for_app_enriched)
- POST   grant membership — 201 with enriched DTO
         409 on duplicate (promotions go through PATCH on purpose so
         the UI can surface "already a member" cleanly)
         422 if the target user is deactivated
         422 if the target's instance_role isn't `member` — owners and
         admins already have implicit authority, so an explicit row
         would be dead weight
- PATCH  change role — 200 with enriched DTO
         404 if no existing membership (use POST to create)
- DELETE remove — 204, idempotent (matches the repo's `remove`
         contract; 204 also when the row never existed)

All four gated on `Capability::AppAdmin(app_id)`. Editors and viewers
get 403 from list and never see the dashboard's Members tab.

No last-app-admin guard: owners implicitly satisfy AppAdmin via
`role_grants`, so removing the last explicit app_admin row cannot
permanently orphan an app — an owner can always re-issue grants.

Wires through picloud/src/lib.rs by splitting the Postgres app_members
repo Arc into two trait views (AppMembersRepository for CRUD, AuthzRepo
for the existing capability lookups) without re-instantiating against
the pool.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/manager-core/src/lib.rs

@@ -12,6 +12,7 @@ pub mod api_key_repo;
 pub mod api_keys_api;
 pub mod app_bootstrap;
 pub mod app_domain_repo;
+pub mod app_members_api;
 pub mod app_members_repo;
 pub mod app_repo;
 pub mod apps_api;
@@ -45,6 +46,7 @@ pub use api_key_repo::{
 pub use api_keys_api::{api_keys_router, ApiKeysState};
 pub use app_bootstrap::{seed_hello_world_if_fresh, HelloWorldOutcome};
 pub use app_domain_repo::{AppDomainRepository, NewAppDomain, PostgresAppDomainRepository};
+pub use app_members_api::{app_members_router, AppMembersApiError, AppMembersState};
 pub use app_members_repo::{
     AppMembersRepository, AppMembersRepositoryError, AppMembershipDetail, AppMembershipRow,
     PostgresAppMembersRepository,