apps_api.rs and app_members_api.rs each grew a near-identical local
`resolve_app` that parses an id-or-slug param and translates None into
their own AppNotFound variant. Promote the lookup half to
`app_repo::resolve_app` (returns `Result<Option<AppLookup>, ...>`) and
let callers handle the None → not-found mapping.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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