[bug] Requestable item cast cards do not open person details

[zZebrahz]

Problem

Cast cards on requestable movie and series detail pages cannot be opened. The same cards link to a person's detail page for titles already in the library, where users can read the person's bio and browse their filmography.

Steps to reproduce

1. Open a movie or series that is not in the library and can be requested.
2. Scroll to the Cast section.
3. Click a cast member's photo or name.

Expected

The cast card should open a person detail page with the available biography and filmography, consistent with cast cards on library item detail pages.

Actual

The cast cards have no link and clicking them does nothing.

Scope

This affects both movie and series request detail pages. Library item detail pages are unaffected because their cast records include Silo person IDs.

Technical notes

• TMDB's credits response includes each cast member's external id, but normalizeCast drops it when building the request-domain cast list.
• RequestMediaCastMember exposes the name, character, profile path, and order, but no internal or external person identifier.
• adaptRequestCast explicitly sets person_id: "".
• CastCarousel only builds a person link when person_id is present, so request cast cards render as non-interactive elements.
• The person detail route currently accepts a Silo person ID. Supporting people who do not yet exist in the local catalog may require resolving the TMDB person ID or adding an external-person detail path rather than assuming a local row exists.

Acceptance criteria

• Cast members on requestable movie and series detail pages can be opened.
• The destination shows the available person biography and filmography.
• External cast identity is preserved through the request service and v2 response.
• Existing local-person links on library item detail pages continue to work.
• Tests cover cast navigation for requestable movies and series, including a person who does not already exist in the local catalog.